Method for partially encrypting program data

ABSTRACT

A cable television system provides conditional access to services. The cable television system includes a headend from which service “instances,” or programs, are broadcast and a plurality of set top units for receiving the instances and selectively decrypting the instances for display to system subscribers. The service instances are partially-encrypted using public and/or private keys provided by service providers or central authorization agents. Keys used by the set tops for selective decryption may also be public or private in nature, and such keys may be reassigned at different times to provide a cable television system in which piracy concerns are minimized.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 09/930,901 filed Aug. 16,2001, which is a continuation of application Ser. No. 09/487,076, filedJan. 19, 2000, now U.S. Pat. No. 6,292,568, which is a continuation ofapplication Ser. No. 09/126,783, filed Jul. 31, 1998, presentlyabandoned, which claims the benefit of U.S. Prov. App. No. 60/054,575,filed Aug. 1, 1997; and is a CIP of application Ser. No. 09/111,958,filed Jul. 8, 1998, now abandoned, which claims the benefit of U.S.Prov. App. No. 60/054,578, filed Aug. 1, 1997; and is CIP of applicationSer. No. 08/767,535, filed Dec. 16, 1996, now U. S. Pat. No. 6,005,938;and is a CIP of application Ser. No. 08/580,759 filed Dec. 29, 1995, nowU.S. Pat. No. 5,870,474, which claims the benefit of U.S. Prov. App. No.60/007,962, filed Dec. 4, 1995; and is CIP of application Ser. No.08/415,617, filed Apr. 3, 1995, now U.S. Pat. No. 5,742,677.

The present application descends from an application, which was one ofseven original applications with identical Detailed Descriptions. All ofthese applications have the same filing date and the same assignee. Theserial numbers and filing dates of the six applications follow:

-   -   Ser. No. 09/127,352, filed Jul. 31, 1998, presently abandoned,        for which a continuation Ser. No. 09/488,230 was filed on Jan.        20, 2000, which issued as U.S. Pat. No. 6,252,964, and        continuation Ser. No. 09/811,085 was filed on Mar. 16, 2001,        which issued as U.S. Pat. No. 6,516,412, and continuation Ser.        No. 10/287,913 was filed on Nov. 5, 2002, currently pending;    -   Ser. No. 09/126,921, filed Jul. 31, 1998, which issued as U.S.        Pat. No. 6,157,719, for which a continuation Ser. No. 09/135,615        was filed on Aug. 18, 1998, which issued as U.S. Pat. No.        6,424,714;    -   Ser. No. 09/127,273, filed Jul. 31, 1998, presently abandoned,        for which a continuation Ser. No. 09/493,409 was filed on Jan.        28, 2000, which issued as U.S. Pat. No. 6,560,340, and for which        continuation Ser. No. 10/377,416 was filed on Mar. 3, 2003,        which is currently pending;    -   Ser. No. 09/127,152, filed Jul. 31, 1998, presently abandoned,        for which a continuation Ser. No. 09/488,104 was filed on Jan.        20, 2000, which issued as U.S. Pat. No. 6,246,767; for which        continuation Ser. No. 09/748,313 was filed on Dec. 26, 2000,        which issued as U.S. Pat. No. 6,526,508; and for which        continuation Ser. No. 09/881,428 was filed on Jun. 14, 2001,        currently pending;    -   Ser. No. 09/126,888, filed Jul. 31, 1998, presently abandoned,        for which a continuation Ser. No. 09/464,794 was filed on Dec.        16, 1999, which issued as U.S. Pat. No. 6,424,717; and    -   Ser. No. 09/126,795, filed Jul. 31, 1998, which issued as U.S.        Pat. No. 6,105,134.

FIELD OF THE INVENTION

The invention concerns systems for protecting and transmittinginformation and more particularly concerns systems for encrypting theinformation that is transmitted against unauthorized access.

BACKGROUND OF THE INVENTION

One way of distributing information is to broadcast it, that is, toplace the information on a medium from which it can be received by anydevice that is connected to the medium. Television and radio arewell-known broadcast media. If one wishes to make money by distributinginformation on a broadcast medium, there are a couple of alternatives. Afirst is to find sponsors to pay for broadcasting the information. Asecond is to permit access to the broadcast information only to thosewho have paid for it. This is generally done by broadcasting theinformation in scrambled or encrypted form. Although any device that isconnected to the medium can receive the scrambled or encryptedinformation, only the devices of those users who have paid to haveaccess to the information are able to unscramble or decrypt theinformation.

A service distribution organization, for example a CATV company or asatellite television company, provides its subscribers with informationfrom a number of program sources, that is, collections of certain kindsof information. For example, the History Channel is a program sourcethat provides television programs about history. Each program providedby the History Channel is an “instance” of that program source. When theservice distribution organization broadcasts an instance of the programsource, it encrypts or scrambles the instance to form encryptedinstance. An encrypted instance contains instance data, which is theencrypted information making up the program.

An encrypted instance is broadcast over a transmission medium. Thetransmission medium may be wireless or it may be “wired”, that is,provided via a wire, a coaxial cable, or a fiber optic cable. It isreceived in a large number of set top boxes. The function of set-top boxis to determine whether encrypted instance should be decrypted and, ifso, to decrypt it to produce a decrypted instance comprising theinformation making up the program. This information is delivered to atelevision set. Known set top boxes include decryptors to decrypt theencrypted instance.

Subscribers generally purchase services by the month (though a servicemay be a one-time event), and after a subscriber has purchased aservice, the service distribution organization sends the set top boxbelonging to the subscriber messages required to provide theauthorization information for the purchased services. Authorizationinformation may be sent with the instance data or may be sent via aseparate channel, for example, via an out-of-band RF link, to a set topbox. Various techniques have been employed to encrypt the authorizationinformation. Authorization information may include a key for a serviceof the service distribution organization and an indication of whatprograms in the service the subscriber is entitled to watch. If theauthorization information indicates that the subscriber is entitled towatch the program of an encrypted instance, the set-top box decrypts theencrypted instance. It will be appreciated that “encryption” and“scrambling” are similar processes and that “decryption” and“descrambling” are similar processes; a difference is that scramblingand descrambling are generally analog in nature, while encryption anddescription processes are usually digital.

The access restrictions are required in both analog and digital systems.In all systems, the continued technological improvements being used toovercome the access restrictions require more secure and flexible accessrestrictions. As more systems switch from an analog format to a digitalformat, or a hybrid system containing both analog and digital formats,flexible access restrictions will be required.

Restricting access to broadcast information is even more important fordigital information. One reason for this is that each copy of digitalinformation is as good as the original; another is that digitalinformation can be compressed, and consequently, a given amount ofbandwidth carries much more information in digital form; a third is thatthe service distribution organizations are adding reverse paths whichpermit a set-top box to send a message to the service distributionorganization, thereby permitting various interactive services.

Thus, the service distribution organizations require access restrictionsthat are both more secure and more flexible than those in conventionalsystems

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of a conditional access system;

FIG. 2A is a block diagram of the service instance encryption techniquesdisclosed herein;

FIG. 2B is a block diagram of the service instance decryption techniquesdisclosed herein;

FIG. 3 is a more detailed block diagram of the service instanceencryption and decryption techniques disclosed herein;

FIG. 4 is a block diagram of the techniques used to dynamically provideentitlement agents to a DHCT;

FIG. 5 is a block diagram of a digital broadband delivery system inwhich the conditional access system is implemented;

FIG. 6 is a block diagram of the conditional access system in thedigital broadband delivery system of FIG. 5;

FIG. 7 is a diagram of an MPEG-2 transport stream;

FIG. 8 is a diagram of how EMMs are mapped into an MPEG-2 transportstream;

FIG. 9 is a diagram of how EMMs are mapped into an IP packet;

FIG. 10 is a diagram of how ECMs are mapped into a MPEG-2 transportstream;

FIG. 11 is a detailed diagram of an EMM.

FIG. 12 is a detailed diagram of a preferred embodiment of DHCTSE 627;

FIG. 13 is a diagram of the contents of memory in DHCTSE 627;

FIG. 14 is a diagram of how NVSCs are allocated to entitlement agents ina preferred embodiment;

FIG. 15 is a diagram of an EAD NVSC;

FIG. 16 is a diagram of other kinds of NVSCs;

FIG. 17 is a diagram of an event NVSC;

FIG. 18 is a diagram of a global broadcast authenticated message (GBAM);

FIG. 19 is a detail of the contents of one kind of GBAM;

FIG. 20 is a diagram showing how GBAMs may be used generally to providedata to a client application;

FIG. 21 is a diagram of a forwarded purchase message;

FIG. 22 is a diagram of the entitlement unit message in an ECM;

FIG. 23 is a diagram of a code message;

FIG. 24 is a diagram showing the relationship between TEDs and the restof conditional access system 601;

FIG. 25 is a detailed diagram of a TED;

FIG. 26 is an illustration of the coordinate system used for spotlightand blackout;

FIG. 27 shows how an area is computed in the coordinate system of FIG.26;

FIG. 28 is a description of a public key hierarchy; and

FIG. 29 is a description of an EMM generator according to the presentinvention.

The reference numbers in the drawings have at least three digits. Thetwo rightmost digits are reference numbers within a figure; the digitsto the left of those digits are the number of the figure in which theitem identified by the reference number first appears. For example, anitem with reference number 203 first appears in FIG. 2.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The following Detailed Description will first provide a generalintroduction to a conditional access system and to encryption anddecryption, will then describe how service instance encoding anddecoding is done in a preferred embodiment, and will thereupon describethe techniques used in the preferred embodiment to authenticate the ECMsand EMMs of the preferred embodiment. Next, the Detailed Descriptionwill describe how EMMs can be used to dynamically add and remove accessto services and the role of encryption and authentication in theseoperations. Finally, there will be a detailed exposition of how thetechniques described in the foregoing are employed in a broadcast datadelivery system with a node structure and a reverse path from the settop box to the head end, of how secure processors and memory areemployed in the preferred embodiment to protect keys and entitlementinformation, and of how certain operations are performed in thepreferred embodiment.

Conditional Access System Overview

FIG. 1 provides an overview of a system 101 for limiting access tobroadcast information. Such systems will be termed herein as“conditional access systems.” A service distribution organization 103,for example a CATV company or a satellite television company, providesits subscribers with information from a number of services, that is,collections of certain kinds of information. For example, the HistoryChannel is a service that provides television programs about history.Each program provided by the History Channel is an “instance” of thatservice. When the service distribution organization broadcasts aninstance of the service, it encrypts or scrambles the instance to formencrypted instance 105.

Encrypted instance 105 contains instance data 109, which is theencrypted information making up the program, and entitlement controlmessages (ECM) 107. The entitlement control messages contain informationneeded to decrypt the encrypted portion of the associated instance data109. A given entitlement control message is sent many times per second,so that it is immediately available to any new viewer or a service. Inorder to make decryption of instance data 109 even more difficult forpirates, the content of the entitlement control message is changed everyfew seconds, or more frequently.

Encrypted instance 105 is broadcast over a transmission medium 112. Themedium may be wireless or it may be “wired”, that is, provided via awire, a coaxial cable, or a fiber optic cable. It is received in a largenumber of set top boxes 113(0 . . . n), each of which is attached to atelevision set. It is a function of set-top box 113 to determine whetherencrypted instance 105 should be decrypted and if so, to decrypt it toproduce decrypted instance 123, which is delivered to the televisionset. As shown in detail with regard to set top box 113(0), set top box113 includes decryptor 115, which uses a control word 117 as a key todecrypt encrypted instance 105. Control word 117 is produced by controlword generator I 19 from information contained in entitlement controlmessage 107 and information from authorization information 121 stored inset-top box 113. For example, authorization information 121 may includea key for the service and an indication of what programs in the servicethe subscriber is entitled to watch. If the authorization information121 indicates that the subscriber is entitled to watch the program ofencrypted instance 105, control word generator 119 uses the key togetherwith information from ECM 107 to generate control word 117. Of course, anew control word is generated for each new ECM 107.

The authorization information used in a particular set top box 113(i) isobtained from one or more entitlement management messages 111 addressedto set top box 113(i). Subscribers generally purchase services by themonth (though a service may be a one-time event), and after a subscriberhas purchased a service, service distribution organization 103 sends settop box 113(i) belonging to the subscriber entitlement managementmessages 111 as required to provide the authorization information 121required for the purchased services. Entitlement management messages(EMMs) may be sent interleaved with instance data 109 in the samefashion as ECMs 107, or they may be sent via a separate channel, forexample via an out-of-band RF link, to set top box 113(i), which storesthe information from the entitlement management message (EMM) 111 inauthorization information 121. Of course, various techniques have beenemployed to encrypt entitlement management messages 111.

Encryption and Decryption Generally

The encryption and decryption techniques used for service instanceencoding and decoding belong to two general classes: symmetrical keytechniques and public key techniques. A symmetrical key encryptionsystem is one in which each of the entities wishing to communicate has acopy of a key; the sending entity encrypts the message using its copy ofthe key and the receiving entity decrypts the message using its copy ofthe key. An example symmetrical key encryption-decryption system is theDigital Encryption Standard (DES) system. A public key encryption systemis one in which each of the entities wishing to communicate has its ownpublic key-private key pair. A message encrypted with the public key canonly be decrypted with the private key and vice-versa. Thus, as long asa given entity keeps its private key secret, it can provide its publickey to any other entity that wishes to communicate with it. The otherentity simply encrypts the message it wishes to send to the given entitywith the given entity's public key and the given entity uses its privatekey to decrypt the message. Where entities are exchanging messages usingpublic key encryption, each entity must have the other's public key. Theprivate key can also be used in digital signature operations, to provideauthentication. For details on encryption generally and symmetrical keyand public key encryption in particular, see Bruce Schneier, AppliedCryptography, John Wiley and Sons, New York, 1994.

The design of an encryption system for a given application involves anumber of considerations. As will be seen in the following,considerations that are particularly important in the broadcast messageenvironment include the following:

-   -   key security: A symmetrical key system is useless if a third        party has access to the key shared by the communicating parties,        and a public key system is also useless if someone other than        the owner of a given public key has access to the corresponding        private key    -   key certification: how can the recipient of a key be sure that        the key he or she has received is really a key belonging to the        entity to which the recipient wishes to send an encrypted        message and not a key belonging to another entity which wishes        to intercept the message    -   message authentication: how can the recipient of a message be        sure that the message is from the party it claims to be from,        and/or that the message has not been altered    -   speed of encryption and decryption: in general, symmetrical key        encryption systems are faster than public key encryption systems        and are preferred for use with real-time data    -   key size: in general, the longer the key used in an encryption        system, the more resources will be required to break the        encryption and thereby gain access to the message

All of the foregoing considerations are influenced by the fact that theenvironment in which a conditional access system operates must bepresumed to be hostile. Many customers of broadcast services see nothingwrong with cheating the service provider and have nothing againsttampering physically with the portion of the conditional access systemthat is contained in the receiver or using various cryptographic attacksto steal keys or to deceive the receiver about the source of themessages it receives. Moreover, the providers of the systems thatactually broadcast the services do not necessarily have the sameinterests as the providers of the service content, and therefore need tocontrol not only who can access a given instance of a service, but alsowhat entities can offer services to a given receiver.

Service Instance Encryption and Decryption: FIGS. 2A and 2B

In overview, the encryption system of the present invention usessymmetrical key encryption techniques to encrypt and decrypt the serviceinstance and public key encryption techniques to transport a copy of oneof the keys used in the symmetrical key techniques of the key from theservice provider to the set-top box.

In FIG. 2A, clear services such as the elementary digital bit streams,which comprise MPEG-2 programs, are sent through a 1^(st) levelencryption called the Program Encrypt function 201, which is preferablya symmetric cipher such as the well-known DES algorithm. Each elementarystream may be individually encrypted and the resulting encrypted streamsare sent to MUX 200 to be combined with other elementary streams andprivate data, such as conditional access data. The key used in theProgram Encrypt function 201 is called the Control Word (CW) 202. The CW202 is generated by control word Generator 203 which can be either aphysically random number generator or can use a sequential counter witha suitable randomization algorithm to produce a stream of random CWs. Anew CW is generated frequently, perhaps once every few seconds and isapplied to each elementary stream on the same time scale. Each new CW isencrypted by Control Word Encrypt & Message Authenticate function 204using a Multi-Session key (MSK) 208 provided by Multi-Session Keygenerator 205. The CW is then combined into an ECM 107 with otherservice-related information. The ECM 107 is authenticated by ControlWord Encrypt & Message Authenticate function 204 that produces a messageauthentication code using a keyed-hash value derived from the messagecontent combined with a secret that can be shared with the receivingset-top box 113. This secret is preferably part or all of the MSK 208.The message authentication code is appended to the rest of the ECM 107.The CW 202 is always encrypted before being sent along with the otherparts of the ECM to MUX 200. This encryption is preferably a symmetriccipher such as the Triple-DES algorithm using two distinct 56-bit keys(which taken together comprise MSK 208).

The MSK 208 has a longer lifetime than CW 202. The MSK lifetime istypically hours to days in length. MSK 208 is both encrypted anddigitally signed by MSK Encrypt & Digital Signature function 206 beforebeing sent to MUX 200 encapsulated in EMM 111. MSK 208 and other partsof EMM 111 are preferably encrypted using a public key algorithm, suchas the well-known RSA algorithm, with a public key associated with thespecific set-top box 113 to which the EMM is addressed. The public keysof all set-top boxes 113 in a system 101 are stored in Public Key DataBase 207. The public keys in this database are preferably certified by acertificate authority. The digital signature function in 206 ispreferably the RSA digital signature method, although others could beused. In the case of an RSA digital signature, the private key, which isused to make the signature, belongs to the entitlement agent withinservice distribution organization 103 responsible for authorizing theassociated service.

In FIG. 2B, the corresponding DHCT private key and associated DHCTpublic secure micro serial number are stored in memory 232 of decoder240. Public secure micro serial number is provided so that demultiplexer230 can select an encrypted multi-session key addressed to decoder 240from transport data stream (TDS). Encrypted multi-session key E_(Kpr)(MSK) is decrypted in decryptor 234 using DHCT private key from memory232 to provide multi-session key MSK. Demultiplexer 230 also selectsfrom transport data stream TDS encrypted control word (CW) E_(MSK) (CW).The encrypted CW is processed in decryptor 236 using multi-session keyMSK as the decryption key to provide the unencrypted CW. The unencryptedCW preferably changes at a high rate, for example, once every fewseconds. Demultiplexer 230 also selects from transport data stream TDSencrypted service E_(CW) (SERVICE). The encrypted service is processedin decryptor 238 using the CW as the decryption key to recover theunencrypted service.

Detailed Implementation of the Encryption System of FIG. 2: FIG. 3

FIG. 3 presents more details about a preferred implementation of thesystem of FIG. 2. Encryption/decryption system 301 has two maincomponents: service origination component 305 and service receptioncomponent 333. The two are connected by a transmission medium 331, whichmay be any medium that will carry a message from service originationcomponent 305 to service reception component 333. Service receptioncomponent 333 is implemented in a set-top box, termed hereinafter adigital home communications terminal (DHCT). It may, however beimplemented in any device which has the necessary computation power, forexample, a personal computer or work station or an “intelligent”television set. In the service origination component, at least theportion labeled 306 is typically implemented in equipment located at thehead end of a broadcasting system such as a cable television (CATV) orsatellite TV system. In some embodiments, however, the head end may beprovided with already-encrypted instances of the service. The remainingportion 308 may also be located at the head end, but may also be locatedanywhere which has access of some kind to head end 306 and servicereception component 333. The latter is particularly the case if the EMMsare sent out of band, for example by way of a wide-area network such asthe Internet. Also, the transmission medium may be storage media, wherethe service origination point is the manufacturer of the media, and theservice reception component may be the element that reads the storagemedia. For example, the transmission medium can be a CD-ROM, DVD, floppydisk, or any other medium that can be transferred, physically,electronically, or otherwise.

Beginning with service origination portion 305, random number generator307 is used to generate MSK 309. Next, an EMM 315 containing MSK 309 andrelated information is produced. EMM 315 also includes a sealed digest.The sealed digest has two purposes: to ensure that the informationplaced in EMM 315 by service origination 305 is the same informationthat arrives at DHCT 333 and to ensure that the information has in factcome from an entity which is empowered to give access to the service.

The sealed digest is made in two stages: first, a digest of the EMM'scontents (here, MSK 309 and the related information) is made by hashingthe contents in a secure one-way hash function to produce a relativelyshort bit string. The secure one-way hash function has three properties:

-   -   the contents that were hashed to produce the short bit string        cannot be determined from the short bit string;    -   any change in what is hashed produces a change in the short bit        string; and    -   it is computationally infeasible to construct a different        message which produces the same short bit string as the EMM.

The short bit string output of the hash function can thus be used todetermine whether the contents of the EMM have changed in transitwithout disclosing those contents. The preferred embodiment uses theMessage Digest 5 one-way hash function, as indicated by the notationMD5. For details on one-way hash functions, see the Schneier reference,supra. The digest is a sealed digest because it is encrypted with aprivate key SP Kr 310 belonging to the entitlement agent (EA) that hasthe right to give the DHCT access to the service for which the MSK isused to produce the key. Before the sealed digest can be used to checkwhether the EMM was transmitted correctly, it must be decrypted usingthe entitlement agent's public key. The sealed digest thus confirms tothe DHCT both that the contents of the EMM have been transmittedcorrectly and that the source of the EMM is the entitlement agent.

Once the sealed digest is made, the contents of the EMM (here, MSK 309and the related information) are encrypted with the public key DHCT Ku312 of the DHCT 333 to which EMM 315 is addressed and EMM 315,containing the encrypted contents and the sealed digest, is sent viatransmission medium 331 to the DHCT 333. In the following, the notationKr is used to indicate a private key and Ku is used to indicate a publickey. The notation RSA indicates that the encryption is done using thewell-known RSA public key encryption algorithm.

As shown in DHCT 333, EMM 315 can only be decrypted by the DHCT 333whose private key 337 (DHCT Kr) corresponds to the public key used toencrypt EMM 315. DHCT 333 decrypts EMM 315 and uses the sealed digest todetermine whether the EMM 315 was correctly transmitted. Thedetermination is made by using public key SP Ku 335 for the entitlementagent to decrypt the sealed digest. Then the contents of EMM 315 arehashed using the same secure one-way hash function that was used to makethe digest. If the results of this hash are identical to the decryptedsealed digest, the determination succeeds. The check with the sealeddigest will fail if the transmission to the DHCT 333 was corrupted intransit, if DHCT 333 does not have the private key corresponding to thepublic key used to encrypt the EMM (i.e., is not the DHCT 333 for whichEMM 315 was intended), or if DHCT 333 does not have public key 335 (SPKu) corresponding to the private key of the EA that was used to make thesealed digest. The latter will be the case if that DHCT 333 has not beengiven access to services provided by the entitlement agent. EMMs 315addressed to DHCT 333 are sent repeatedly; consequently, if the problemwas corruption in transit, an uncorrupted EMM 315 will be receivedshortly and the determination will succeed. How DHCT 333 comes to haveSP Ku 335 needed to decrypt the sealed digest will be explained in moredetail later.

The next stage in service origination 305 is generating control word 319used to actually encrypt service instance 325 and generating the ECM 323that carries the information needed to decrypt the service instance toDHCT 333. The control word 319 is generated by random number generator317. This can be a true random number generator, whose output is theresult of some basic underlying random physical process, or some othermeans, for example, the result of encrypting a value, called a “counter”(which increments by one after each use) with 3DES, using the MSK as thekey. In the case of a true random number, the encrypted control word istransmitted in the ECM. In the case of the counter-based control wordgeneration, the clear version of the “counter” is used in thetransmitted ECM. As mentioned above, the control word is a short-termkey, i.e, it has a life time of a few seconds or less. Included in theECM 323 is a digest of the contents plus the MSK that is made using theMD5 one-way hash just described. The inclusion of the MSK in making thedigest gives the entitlement agent to which the ECM 323 belongs a sharedsecret with the DHCTs 333 that are entitled to receive service instancesfrom the entitlement agent and consequently prevents “spoofing” of ECMs323, that is, provision of ECMs 323 from a source other than theentitlement agent. As will be seen in more detail later, the preferredembodiment uses the shared secret technique generally to authenticatemessages that contain messages that have real-time value with regard toan instance of a service.

ECM 323 is sent together with encrypted content 329 to DHCT 333. Thefirst ECM 323 for a given portion of encrypted content 329 must ofcourse arrive at DHCT 333 before the encrypted content does. In thepreferred embodiment, content 325 and ECM 323 are encoded according tothe MPEG-2 standard. The standard provides for a transport stream thatincludes a number of component streams. Some of these carry content 329,another carries the ECMs 323, and a third carries the EMMs 315. Only thestreams carrying content 329 are encrypted according to DES 329; sincethe control words in ECMs 323 and the contents of EMMs 315 have alreadybeen encrypted, no further encryption is needed when they are sent inthe MPEG-2 transport stream. The manner in which EMMs and ECMs aretransported in the MPEG-2 transport stream will be described in moredetail later.

When an ECM 323 is received in DHCT 333, control word 319 is eitherdecrypted or found by encrypting the counter value at 343 using the MSK.The integrity of the contents of the ECM 323 is checked by comparing thevalue resulting from hashing the contents plus some or all of the MSK(based on cryptographic principles) in the one-way hash function withthe message digest contained in ECM 323. Included in the contents arecontrol word 319 and information identifying the service instance 325which ECM 323 accompanies. The identifying information is used togetherwith the authorization information received with EMM 315 to determinewhether DHCT 333 is authorized to receive the service instance 325. Ifit is, control word 3 19 is used in service decryptor 347 to decryptencrypted content to produce original content 325.

System 301 offers a number of advantages with regard to security. Ittakes advantage of the speed of symmetrical encryption systems wherethat is needed to decrypt encrypted content 329 and the control word inECM 323. Encrypting it using the MSK protects the control word, and ECM323 is authenticated by using some or all of MSK 309 as a shared secretbetween the entitlement agent and DHCT 333. MSK 309 is protected in turnby the fact that it is sent in an EMM which is encrypted using theDHCT's public key and by the fact that the EMM includes a sealed digestwhich is encrypted using the entitlement agent's private key. Furthersecurity is provided by the fact that service identification informationfrom ECM 323 must agree with the authorization information received inEMM 315 before control word 319 is provided to service decryptor 347.For example, as described in detail in the Banker and Akins parentpatent application supra, one use of the information in ECM 323 and EMM315 is to prevent what are termed “replay attacks” on the encryptedservices. In addition to being secure, system 301 is flexible. Theauthorization information contained in EMM 315 and the serviceidentification information contained in ECM 323 together permit a widerange of access to service instances received in DHCT 333.

Dynamic Provision of Multiple Entitlement agents to DHCT 333: FIG. 4

The use of the sealed digest in EMM 315 means that DHCT 333 will notrespond to EMM 315 unless it has a public key for the entitlement agentthat has the power to give entitlements to the service to be decryptedby the MSK in EMM 315. This is part of a broader arrangement which makesit possible to dynamically provide DHCT 333 with one or more entitlementagents and to dynamically remove provided entitlement agents from DHCT333.

The entity that provides and removes entitlement agents is called theconditional access authority (CAA). The arrangement further permitsentitlement agents that have been provided to DHCT 333 to dynamicallymodify their authorization information in DHCT 333. All of theinformation needed to perform these operations is sent via EMMs, withthe sealed digests being used to ensure that only the CAA may add orremove entitlement agents and that only the entitlement agent to whichauthorization information belongs may modify the authorizationinformation.

The above arrangement has a number of advantages:

-   -   It permits multiple entitlement agents.    -   It permits dynamic addition and removal of entitlement agents.    -   It places limits on the services to which an entitlement agent        may grant entitlements, but otherwise permits entitlement agents        to manage their own authorization information.    -   It separates the business of providing entitlements to services        and service instances from the business of actually providing        instances of the service; consequently, a CATV operator may        simply run as a distribution utility.    -   It separates the business of giving an entity the right to be an        entitlement agent from the business of being an entitlement        agent.    -   It provides an easy way of permitting a customer to change        entitlement agents as he or she sees fit.    -   It provides a secure arrangement whereby a DHCT 333 may        communicate by means of a reverse path with an entitlement        agent, a conditional access authority, or potentially the        provider of the instances of the service.

FIG. 4 shows how the arrangement is implemented in a preferredembodiment. FIG. 4 is best understood as an extension of FIG. 3. BothFIG. 4 and FIG. 3 have the same major components: service origination305, DHCT 333, and transmission medium 331 for coupling the two.Further, encryptor 313 and decryptor 339 are used in both figures.Moreover, as indicated by reference number 308, the EMMs may be eithersent together with a service instance or by another channel. FIG. 4further shows an additional component of DHCT 333, namely EMM manager407. EMM manager 407 is implemented in software executed in a secureprocessor in DHCT 333. The task of EMM manager 407 is to respond to EMMsthat add or remove entitlement agents and to EMMs that modify theauthorizations for an entitlement agent. EMM manager 407 furtherprovides messages by means of which DHCT 333 may communicate with anentitlement agent or a conditional access authority.

Initially, EMMs that modify an entitlement agent's authorizationinformation are made in response to modification information 403provided by the entitlement agent or required by the network operator.As shown at 313, the modification information is encrypted using thepublic key 312 for DHCT 333 and has a sealed digest that is encryptedusing the private key 310 for the entitlement agent. The resultingauthorization modification EMM 405 is sent via transmission medium 331to decryptor 339 in DHCT 333, where it is decrypted and checked in themanner described above for EMMs 315 containing an MSK. The EAmodification information 403 contained in the EMM goes, however, to EMMmanager 407, which uses the information to modify the authorizationinformation for the entitlement agent in DHCT 333. Examples ofmodifications include adding or canceling services provided by theentitlement authority and changing the conditions under which access toinstances of a given service will be granted.

As indicated above, the sealed digest is encrypted using the private keyof the entitlement agent. Consequently, the validity of the EMM can onlybe determined if DHCT 333 has the entitlement agent's public key. Thepublic key for an entitlement agent is provided to DHCT 333 by an EAallocation EMM 413 from a conditional access authority. EMM 413 containsentitlement agent allocation information 409 from the conditional accessauthority; at a minimum, entitlement agent allocation information 409contains the public key for the entitlement agent; it may also containinformation about the amount of memory an entitlement agent may have inDHCT 333 and about classes of service that an entitlement agent mayoffer. For example, the entitlement agent may not be permitted to offerinteractive services. Information 409 is encrypted with the public key312 of DHCT 333, and the sealed digest is encrypted with private key 411of the conditional access authority.

In DHCT 333, EMM 413 is decrypted using private key 337 belonging toDHCT 333 and the sealed digest is decrypted using CAA public key 415. Ifthe digest confirms the correctness of the contents of the EMM, EMMmanager 407 allocates storage for the entitlement agent whose public keyis contained in EMM 413. That done, EMM manager 407 places theentitlement agent's public key in the storage. The storage provides aplace to store the entitlement agent's public key, the authorizationinformation for the services and service instances provided by theentitlement agent, and the MSKs provided by the entitlement agent. OnceDHCT 333 has the entitlement agent's public key and storage for theentitlement agent's authorization information and MSK, EMM manager 407can respond to EMMs from the entitlement agent. Of course, in order todecrypt the sealed digest, DHCT 333 must have public key 415 for theconditional access authority. As will be explained in more detail lateron, in a preferred embodiment, public key 415 and the public and privatekeys for DHCT 333 are installed in DHCT 333 at the time that DHCT 333 ismanufactured.

When a customer orders a service, the arrangements just describedinteract as follows:

-   -   1. If the service is provided by an entitlement agent for which        the customer's DHCT 333 does not have the public key, the        conditional access authority must first send EA allocation EMM        413 to DHCT 333; EMM manager 407 responds by allocating storage        for the entitlement agent. Only the conditional access authority        can send EA allocation EMM 413, and consequently, the        conditional access authority (CAA) can control access by        entitlement agents to customers of a particular service        distribution organization.    -   2. If DHCT 333 has the entitlement agent's public key, either        because step (1) has just been performed or was performed at        some time in the past, the entitlement agent sends modification        EMM 405 with the authorization information for the newly ordered        service or service instance to DHCT 333. EMM manager 407        responds thereto by storing the authorization information in the        allocated space.    -   3. Once step (2) is done, DHCT 333 can receive EMM 315 with the        MSK for the service from the entitlement agent. EMM manager 407        stores the MSK in the allocated space.    -   4. When the actual service instance is sent, it is accompanied        by ECMs containing the current control word. The MSK is used to        decrypt the ECMs and the control words obtained from the ECMs        are used to decrypt the instance of the service.

The above use of EMMs and ECMs to control access to instances of aservice thus guarantees that no entitlement agent will have access toDHCT 333 without permission of the conditional access authority and thatno DHCT 333 will have access to an instance of a service withoutpermission of the entitlement agent for the service. It also makes itpossible for the entitlement agent to be in complete control of theservice. The EMMs 405 and 315 define access to the service, and theentitlement agent may send these to DHCT 333 independently of theservice distribution organization. Further, it is the entitlement agentthat provides the MSK used to generate control words and decrypt the ECMto both the service distribution organization and DHCT 333. Indeed, ifthe entitlement agent wishes to do so, it can itself provide encryptedinstances of the services to the service distribution organization,which, in such a case, merely functions as a conduit between theentitlement agent and DHCT 333.

Secure Transmission of Messages Via the Reverse Path

FIG. 4 also shows how the techniques used to ensure the security of EMMsare also used to ensure the security of messages sent from DHCT 333. Theexample shown in FIG. 4 is a forwarded purchase message (FPM). Theforwarded purchase message is used for the interactive purchase of aninstance of a service. One example of such a purchase is what is calledimpulse pay-per-view, or IPPV. In such a system, the beginning of anevent, for example, a baseball game, is broadcast generally andcustomers can decide whether they want to see all of it. In that case,they must provide input to DHCT 333 that indicates that they wish to seethe entire event. EMM manager 407 responds to the input by making theFPM and sending it to the entitlement agent so that the entitlementagent can charge the customer for the event and send an EMM 315confirming that DHCT 333 may continue to decrypt the event. Theinformation needed by the entitlement agent is forwarded entitlementinformation 417; to ensure the privacy of the customer, this informationis encrypted using the 3DES algorithm with a key 420, as shown at 343,to produce encrypted forward entitlement information 419. The key 420 iscomposed of two 56-bit DES keys. The 3DES encryption operation is asequence of three DES operations: encryption using the first DES key,decryption using the second DES key, and encryption using the first DESkey Then key 420 is encrypted using the public key 335 of theentitlement agent and the sealed digest is made using the private key ofDHCT 333. All of these parts together make up forwarded purchase message421, which is addressed to the entitlement agent.

At the entitlement agent, key 420 is decrypted using the entitlementagent's private key 310, and the sealed digest is decrypted using thepublic key 312 of the DHCT. If the Encrypted Forwarded EntitlementInformation (EFEI) 419 contained in the FPM 421 is determined not tohave been tampered with, it is passed to 3DES decryption 443, whichdecrypts it using key 420 and provides forwarded entitlement information417 to the entitlement agent. As will be immediately apparent, the sametechnique, with or without the 3DES encryption of the contents of themessage, can be used to send messages to any entity for which DHCT 333has the public key. At a minimum, this includes the CAA and anyentitlement agent that has been allocated memory in DHCT 333.

Authentication of Global Broadcast Messages

A global broadcast message is one that is not addressed to anyindividual DHCT 333 or to any group of DHCTs 333. In a preferredembodiment, global broadcast messages accompany instances of servicesand contain information that is relevant to the instance they accompany.Consequently, the encryption and authentication techniques used in theglobal broadcast messages must permit rapid decryption and authenticitychecking. One example of a global broadcast message is the ECM. Otherexamples are the different types of global broadcast authenticatedmessages, or GBAMs. As with ECMs, it is necessary to prevent globalbroadcast messages from being spoofed, and it is done in the samefashion as with the ECMs. More specifically, the digest is made usingsome or all of the MSK together with the content of the global broadcastmessage. The MSK thus functions as a shared secret between theentitlement agent and DHCT 333. When EMM manager 407 receives the globalmessage, it makes a digest using the contents of the received messageand the MSK and responds to the received message only if the digestagrees with the one contained in the message. An advantage of using adigest made with the MSK to authenticate the global broadcast message isthat the digest may be both made and checked very quickly.

Implementation of the Conditional Access System in a Digital BroadbandDelivery System

The foregoing has described the conditional access system in terms ofECMs, EMMs, and other messages and in terms of the manner in which themessages and their digests are encrypted and decrypted. The conditionalaccess system as just described will work with any communicationsarrangement which permits an instance of a service to be delivered to aDHCT together with ECMs and other broadcast messages and which permitsthe DHCT to receive EMMs from a conditional access authority and one ormore entitlement agents. The conditional access system is, however,particularly well-suited for use in a modem digital broadband deliverysystem, and the following will describe how the conditional accesssystem is implemented in such a delivery system.

Overview of the Digital Broadband Delivery System: FIG. 5

FIG. 5 provides an overview of digital broadband delivery system (DBDS)501. DBDS 501 includes service infrastructure 503, a headend 515, atransport infrastructure 517, hubs 519(0 . . . n), access networks 521(0. . . n), and Digital Home Communications Terminals (DHCTs) 333. Theservice infrastructure consists of Value-Added Service Provider (VASP)systems 509, which are systems that provide services to the broad banddelivery system, the Digital Network Control System (DNCS) 507, whichmanages and controls services provided by means of DBDS 501, theAdministrative Gateway (AG) 505, which is a source of serviceprovisioning and authorization information in DBDS 501, NetworkManagement System (NMS) 511, which maintains a database of system statusand performance information, and the Core Network 513, whichinterconnects other Service Infrastructure 503 components with headend515. In a preferred embodiment, Core Network 513 consists of ATM-basedswitching and transmission facilities. Headend 515 provides an interfacebetween service infrastructure 503 and transport infrastructure 517.Transport infrastructure 517 provides a high-bandwidth interconnectionfrom headend 515 to hubs 519(0 . . . n). Each hub 519(i) serves anaccess network 521(i), which consists of hybrid fiber coax (HFC) nodes523 connected via a coax bus network to DHCTs 333. A given DHCT 333(k)in DBDS 501 thus belongs to an HFC node 532(j) in an access network521(i). Transport infrastructure 517 and access network 523 may provideonly a forward channel from head end 515 to a given DHCT 333(k), butpreferably provide both a forward channel and a reverse path. Eachinstance of a DBDS 501 generally provides service to a metropolitanarea.

DBDS 501 can be implemented in a variety of configurations to fit thecircumstances of a particular service environment. For example, headendequipment may be deployed within headend 515, within a hub 519(i), or aspart of a VASP system 509. DNCS components 506 may be deployed withinheadend 515 or distributed among the hubs 519. Transport infrastructure517 may utilize SONET add/drop multiplexing, analog fiber technology, orother transmission technologies.

Overview of the Conditional Access System: FIG. 6

FIG. 6 shows the components of a preferred embodiment of conditionalaccess system 601 in DBDS 501. Conditional access system 601 is acollection of components DNCS 507, headend 515, and DHCT 333 thattogether provide security and conditional access services.

The components of conditional access system 601 perform the followingfunctions:

-   -   1. encrypting the service content    -   2. encrypting the control words used for service encryption    -   3. authenticating the ECMs that contain the encrypted control        words    -   4. passing the ECMs to DHCTs    -   5. managing a subscriber authorization database    -   6. encrypting and authenticating EMMs containing subscriber        entitlement information    -   7. passing the EMMs to DHCTs    -   8. decrypting the EMMs and checking their authenticity at the        DHCTs    -   9. responding to the EMMs by modifying entitlement information        in the DHCTs    -   10. responding to the ECMs by authenticating them, decrypting        the control word, and checking entitlement at DHCT 333, and    -   11. if the ECM is authentic and the authorizations permit,        decrypting the service content.

These requirements are met by the following components of conditionalaccess system 601:

-   -   Stream Encryption & ECM Streamer Modules 620 in head end 515;    -   Control Suite 607 in DNCS 507;    -   I. Transaction Encryption Device 605 in head end 515, with        secure link to DNCS 507;    -   II. Service Decryptor Module 625 in DHCT 333;    -   III. Security Manager Module 626 in DHCT 333; and    -   IV. DHCTSE 627 in DHCT 333.

FIG. 6 depicts a typical configuration of these components for securingdigital services within DBDS 501. In the following, the components willbe described in more detail.

Service Encryption & ECM Streamer Module 620

Service Encryption and ECM Streamer (SEES) module 620 is a component ofQAM Modulator 619 that operates under direction of control suite 607 toencrypt the MPEG-2 transport stream packets that are employed in thepreferred embodiment to transmit service content 325. As shown in FIG.6, service content 325 may be received from sources such as a digitalsatellite distribution system 613, a digital terrestrial distributionsystem 611, or a media server 609. Media server 609 may be connected tohead end 515 by a broadband integrated gateway 615. SEES 620 uses MSK309 to generate the control words 319 used for service encryption andcreates ECMs 323 for transporting the control words together withencrypted service content 329 within the outgoing MPEG-2 TransportStream. SEES 620 encrypts the control words in the ECMs 323 with MSKs309. The MSKs are generated by TED 603 and are sent to SEES 620 inencrypted form in EMM-like messages.

DHCT 333

DHCT 333 is connected between the HFC network 521 and the customer'stelevision set. DHCT 333 receives and interprets EMMs, ECMs, and GBAMsand decrypts instances of services. DHCT 333 further provides thecustomer interface for DBDS 501 and receives customer input 628 from thecustomer. In response to the customer input, DHCT 333 may generate FPMsor other messages that travel via the reverse path to the CAA or to EAs.In a preferred embodiment, DHCT 333 is implemented using a combinationof general-purpose processors, ASICs, and secure elements (which may beimplemented discretely or integrated). For purposes of the presentdiscussion, DHCT 333 has three important components: service decryptionmodule 625, security manager 626, and DHCT secure element (DHCTSE) 627.Service decryption module 625 is preferably implemented in an ASIC, andsecurity manager 626 is preferably implemented in software. DHCTSE 627is a secure element for performing security and conditionalaccess-related functions.

Service Decryptor Module 625

Service decryptor module 625 is the component of DHCT 333 that decryptsthe encrypted MPEG-2 transport stream packets. Service decryptor 625receives the control words to be used for service decryption from DHCTSE627. DHCTSE 627 controls which transport stream packets are decrypted byonly passing the control words for authorized services to servicedecryptor 625.

Security Manager 626

Security manager 626 is a software module of the DHCT that provides aninterface between applications running on DHCT 333 that use theconditional access system and DHCTSE 627. It also coordinates processingbetween the service decryptor module and DHCTSE 627.

DHCTSE 627

DHCTSE 627 stores keys, interprets EMMs and ECMs, and produces FPMs.With the EMMs and ECMs, it does the decryption and authenticationrequired for interpretation and with FPMs, it makes the sealed digestand encrypts the FPM. Thus, in the preferred embodiment, EMM manager 407is implemented in secure element 627. In addition, DHCTSE 627 providesencryption, decryption, digest, and digital signature services for otherapplications executing on DHCT 333. Secure element (DHCTSE) 627 includesa microprocessor and memory that only the microprocessor may access.Both the memory and the microprocessor are contained in tamper-proofpackaging. In interpreting EMMs, DHCTSE 627 acquires and stores keys andentitlement information; in interpreting ECMs, DHCTSE 627 uses theentitlement information to determine whether DHCT 333 receiving the ECMhas an entitlement for the instance of the service which the ECMaccompanies; if it does, DHCTSE 627 processes the ECM, and provides thecontrol word to service decryptor module 625 in a form that it may useto decrypt or descramble services. DHCTSE 627 further records purchaseinformation for impulse-purchasable services such as IPPV and stores thepurchase data securely until the data is successfully forwarded via aforwarded purchasing message to control suite 607. DHCTSE 627 maintainsMSK for the EAs, the private/public key pairs for DHCT 333, and thepublic keys of the conditional access authorities and the entitlementagents.

Control Suite 607

Control suite 607 is a member of the DNCS family of software. Controlsuite 607 controls the encryption of services performed by a SEES module620 based upon input from the DNCS broadcast control suite component.Control Suite 607 also maintains a database of subscriber authorizationsbased upon transactions received from Administrative Gateway 511.Control suite 607 generates EMMs for communicating subscriberauthorizations and other conditional access parameters to the DHCTSE627. Control suite 607 acts on behalf of entitlement agents. The EMMsgenerated by control suite 607 for communicating subscriberauthorizations and other conditional access parameters to DHCTSE 627 areencrypted with the public keys of the DHCTs 333 to which they aredirected and are authenticated with the private key of the EA, which ismaintained by transaction encryption device (TED) 603. DHCTSE 627maintains the public key of the EA and uses it to confirm theauthenticity of EMMs generated by control suite 607 for the EA.

Control Suite 607 further enables the establishment of a conditionalaccess authority (CAA). Control suite 607 generates EA allocation EMMs413 which pass the public key of the EA to a DHCTSE 627. These EMMs 413are encrypted as described above, but are authenticated using a digitalsignature made with the private key of the CAA, which is maintained byTED 603. DHCTSE 627 is pre-provisioned with the public key of the CAAfor use in confirming the authenticity these EMMs 413.

Communications between control suite 607 and the rest of conditionalaccess system 601 are by means of LAN interconnect devices 605 and 617.Device 605 l s connects Control Suite 607 to Administrative Gateway 505,from which it receives the information necessary to make ECMs and EMMs,and device 617 connects it to the SEES modules 620 in the QAM modulatorsand to QPSK modulator 621 and QPSK demodulator 623, which are in turnconnected to HFC network 521. The connection between Control Suite 607and DHCT 333 via LAN interconnect device 617, modulator 621, demodulator623, and HFC network 521 implements the reverse path needed for messagessuch as FPM 421 and also implements a forward channel to DHCT 333. Thisforward channel is independent of the forward channel used to providethe services. In conditional access system 601, Control Suite 607 cansend EMMs or broadcast messages to DHCT 333 either by the forwardchannel just described or by sending them together with an instance of aservice.

Transaction Encryption Device 603

Transaction Encryption Device (TED) 603 serves as a peripheral toControl Suite 607. TED 603, under the direction of Control Suite 607,encrypts and makes sealed digests of various conditional access systemmessages, including EMMs. TED 603 may also generate and store (MSKs)which are used by SEES 620 to encrypt the control words in the ECMs andto decrypt the control words in DHCTSE 627. TED 603 further uses theMSKs to authenticate the global broadcast message class of conditionalaccess system messages. Authentication is done by hashing the contentsof the message together with some or all of the MSK. TED 603 decryptsand verifies the authenticity of Forwarded Purchase Messages 421 sentfrom the DHCTs 333 as well as other messages sent using the reversepath. TED 603 maintains the private keys of the CAA and the EA andreceives from the DNCS the public keys of the DHCTs from which itreceives messages. As will be explained in more detail below, TED 603receives the public keys from a source that confirms the authenticity ofeach key. TED 603 finally makes a sealed digest for the EMMs using theprivate key of the CAA and EA as appropriate for the EMM.

Using the Conditional Access System to Support Services and ProgramsExecuting in DHCT 333 or Service Infrastructure 507

The conditional access system can be utilized to secure the provisioningof a service or to provide security services to programs executing onDHCT 333 or programs in Control Suite 607. Secure service provision doesnot require that the DHCT programs that support the service be secure.The reason for this is that the following may be done only by DHCTSE 627in DHCT 333 or by a TED 603:

-   -   generation of the MSK;    -   storage of the MSK;    -   storage of the keys needed to encrypt and/or decrypt EMMs and to        make and check sealed digests;    -   storage of the entitlement information received from the EAs;    -   encryption and/or decryption of EMMs;    -   encryption or decryption of the control word;    -   provisioning of the MSK to SEES module 607 and the decrypted        control word to service decryption module 625;    -   making and checking digests with shared secrets;    -   making and checking sealed digests;    -   confirming that a DHCT 333 is entitled to receive a service.

A program executing on DHCT 333 or a program in control suite 607 has noaccess to any of the information stored in DHCTSE 627 or TED 603 and canthus do nothing with EMMs and ECMs beyond asking DHCTSE 627 or TED 603to generate or interpret them. For example, when DHCT 333 receives anEMM, it simply passes the EMM to DHCTSE 627 for processing; when itreceives an ECM, it does the same; if the authorization informationcontained in the ECM and stored in the DHCTSE 627 indicates that DHCT333 is entitled to the service, DHCTSE 627 provides the decryptedcontrol word to service decryption module 625.

The conditional access system can also do security checking for programsgenerally. For example, a program executing on DHCT 333 that requiresdownloaded information from a server application may expect that asealed digest was added to the information before it was downloaded, andthe program may use DHCTSE 627 to check the sealed digest and determinewhether the information is authentic, but it is up to the program todecide what to do with the information when DHCTSE 627 indicates that itis not authentic.

Details of Messages in Conditional Access System 601

In conditional access system 601, the ECM, the EMM, the FPM, and theGBAM are all different types of conditional access messages. Theconditional access messages all have a common format, namely a header,the message itself, and a message authentication code, or MAC. Theheader contains the following information:

-   -   the type of the message, i.e., whether it is an ECM, EMM, GBAM,        or something else;    -   the length of the message;    -   an identifier for the conditional access system;    -   an identifier for the type of security algorithm used with the        message, including encryption of the message and authentication        of its contents; and    -   the length of the message content.

The header is followed by the encrypted message and the MAC, which,depending on the message type, may be a sealed digest or a digest madewith some or all of the MSK together with the message.

In digital broadband delivery system 501, CA messages may travel eitherin a MPEG-2 data stream or in an IP packet, that is, a packet madeaccording to the rules of the Internet Protocol. Also, other transportprotocols such as ATM may be used. In the preferred embodiment, messagesfrom control suite 607 to DHCT 333 may travel in MPEG-2 or IP packets;messages from DHCT 333 to control suite 607 travel as IP packets on thereverse path provided by QPSK demodulator 623 and LAN interconnectdevice 617. In general, messages to DHCT 333 which are closelyassociated with particular instances of services, such as ECMs andGBAMs, travel in the MPEG-2 data stream; EMMs may travel either in theMPEG-2 transport stream or as IP packets via LAN interconnect device 617and QPSK modulator 621.

CA Messages in the MPEG-2 Transport Stream: FIG. 7

FIG. 7 is a schematic representation of an MPEG-2 transport stream 701.An MPEG-2 transport stream is made up of a sequence of 188-byte longtransport packets 703. The packets 703 in the stream carry informationthat, when combined at DHCT 333, defines an instance of a service andthe access rights of a given DHCT 333 to the service. There are twobroad categories of information: program 709, which is the informationneeded to produce the actual pictures and sound, and program specificinformation (PSI) 711, which is information concerning matters such ashow the transport stream is to be sent across the network, how theprogram 709 is packetized, and what data is used to limit access to theprogram 709. Each of these broad categories has a number ofsubcategories. For example, program 709 may include video informationand several channels of audio information.

Each transport packet 703 has a packet identifier, or PID, and all ofthe packets 703 that are carrying information for a given subcategorywill have the same PID. Thus, in FIG. 7, the packets carrying Video 1all have PID (a), and the packets belonging to that subcategory areidentified by 705(a). Similarly, the packets carrying Audio 1 all havePID (b), and the packets belonging to that category are identified by705(b). A subcategory of information can thus be identified by the PIDof its packets. As shown at output packets 707, the output from MUX 704is a sequence of contiguous individual packets from the varioussubcategories. Any part or all of MPEG-2 transport stream 701 may beencrypted, except that packet headers and adaptation fields are neverencrypted. In the preferred embodiment, the sets of packets making upprogram 709 are encrypted according to the DES algorithm, with thecontrol word as a key.

Two of the subcategories are special: those identified by PID 0 (705(e))and PID 1 (705(c)) list the PIDs of the other packets associated withthe service(s) and thus can be used to find all of the informationassociated with any service. The packets in PID 1 705(c) have as theircontents a conditional access table 710, which lists the PIDs of otherpackets that contain EMMs. One set of such packets appears as EMMpackets 705(d), as indicated by the arrow from CAT 710 to packets705(d). Each packet 703 in packets 705(d) contains private information,that is, information that is private to conditional access system 601.As will be explained in more detail below, private information 713, forthe purposes of this invention, is a sequence of CA messages, each ofwhich contains an EMM, and private information 719, is a sequence ofmessages, each of which contains an ECM.

The packets in PID 0 705(e) contain a program association table whichlists PIDs of packets that are associated with a particular instance ofa service. One such set of packets is program maps packets 705(f), whichcontain a program map table 717 that lists, amongst other things, thePIDs of transport packets 703 containing ECMs for the program. One suchset of packets is shown at 705(g). Each of the transport packetscontains private information 719, which in this case is a sequence of CAmessages, each of which contains an ECM.

FIG. 8 shows in detail how EMMs are carried in transport packets 703.The payload space 719 in the packets carries data from aCA_PRIVATE_SECTION layer 803, which in turn contains a sequence of CAmessages 805, each of which contains an EMM 807. In the sets of packets705(g) carrying ECMs, the control words in the ECMs are encrypted usingthe 3DES algorithm with the MSK as key; in the sets of packets 705(d)carrying EMMs, the EMMs are encrypted using the public key of DHCT 333for which they are intended. As will be immediately apparent, thetechniques just described can be employed to transmit any CA message 805as part of an MPEG-2 transport stream.

Mapping CA Messages into IP Protocol Packets: FIG. 9

FIG. 9 shows how EMMs are mapped into the Internet Protocol (IP) packetsused to communicate between control suite 607 and DHCT 333 via LANdevice 617 and QPSK modulator 621 and demodulator 623. An IP packet 903is a variable-length packet that consists simply of a header and apayload. The header contains source and destination IP addresses for thepacket. With an EMM, the source address is the IP address of the CA orEA, and the destination address is the IP address of DHCT 333. In thepreferred embodiment, the IP address of DHCT 333 is constructed usingits serial number. The IP addresses in DBDS 501 are partitioned by HFCnode 523. The payload of the IP packet is a packet 905 belonging to theUser Datagram Protocol (UDP) which has as its payload aCA_PRIVATE_SECTION 803, which in turn contains a sequence of CA messages805, each of which contains an EMM 807.

ECM Structure Details: FIG. 10

FIG. 10 shows details of the structure of an ECM 1008 and shows themapping 1001 from an ECM 1008 to a set 705(e) of MPEG-2 transportpackets 703. As before, the data of a CA_PRIVATE_SECTION 803 is carriedin a set of MPEG-2 transport packets 703 with the same PID. The data isa header 1003 for private section 803 and a sequence of CA messages 805,each of which includes a CA message header 1005, a CA ECM message 1007,and an ECM MAC 1013. CA ECM message 1007 and ECM MAC 1013 together makeup ECM 1008.

FIG. 10 also shows how the control word is protected in ECM 1008 and howECM MAC 1013 is produced. The control word is a random value that iseither encrypted using 3DES encryption or created by encrypting acounter value using 3DES encryption, using the MSK as the key. In eithercase, the preferred embodiment calls for an MSK which is made up of two56-bit DES keys, and the 3DES encryption operation is a sequence ofthree DES operations: encryption using the first DES key, decryptionusing the second DES key, and encryption using the first DES key. Thecontrol word, too, may have even or odd parity. As shown at 1013, theodd control word (after suitable encryption) becomes part ofECM_entitlement_unit_message 1011, and, in its non-encrypted form, isused together with some or all of the MSK as input to the MD5 one-wayhash function to produce ECM MAC 1013. The same procedure is used withthe even-parity control word. The contents other than the control wordof ECM_entitlement_unit_message 1011 will be examined in more detaillater.

EMM Structure Details: FIG. 11

FIG. 11 shows a CA message 805 that contains an EMM 1112. CA message 805has a header 1003, a CA EMM message 1101, and a sealed digest 1103. CAEMM message 1101 consists of CA EMM message header 1105, EMM message1107, and CRC error detection code 1109. EMM message 1107 in its turncontains EMM header 1113 and EMM_inside_data 1115. EMM_inside_data 115is encrypted using the public key of the DHCT 333 for which it isintended. The data that is encrypted is EMM data 1129, which in turn ismade up of EMM_inside_header 1123 and EMM command_data 1125 togetherwith padding 1127. EMM data 1129 is also input to the MD5 one-way hashfunction to produce EMM MAC 1119 and sealed digest 1103 is made byencrypting EMM_signing_header 1117, EMM MAC 1119, EMM_signing header1117, and padding 1121 with the private key of either an entitlementagent or a conditional access authority, depending on what kind of EMMit is.

The EMM_signing-header is information from the EMM_inside_header. Thisinformation is particularly sensitive and is consequently encrypted byboth the public key of DHCT 333, for privacy reasons, and the privatekey of the entitlement agent or the conditional access authority, toapply a digital signature. Upon reception, and after the privacydecryption, if the signature verification fails, the EMM is discarded byDHCT 333. Included in this information are an ID for the conditionalaccess system, the type of the CA message, the serial number of themicroprocessor in the DHCT's DHCTSE 627, an identifier for the CAA or EAwhich is the source of the EMM, an indication of which of the threepublic keys for the CAA in DHCT 333's secure element is to be used todecrypt the sealed digest, and an indication of the format of the EMM.The contents of EMM command_data 1125 will be explained in more detailin the discussion of the operations performed using EMMs.

Details of DHCTSE 627: FIGS. 12-14

DHCTSE 627 has five main functions in conditional access system 601:

-   -   It securely stores keys including the public and private keys        for DHCT 333, public keys for the CAA, public keys for EAs from        which DHCT 333 is authorized to receive services, and MSKs        provided by those EAs.    -   It securely stores entitlement information sent by the EAs.    -   It decrypts, authenticates, and responds to EMMs.    -   It decrypts the control words in the ECMs, authenticates the        ECMs, and when DHCT 333 is authorized to receive the service        instance to which the ECM belongs, it provides the control word        to service decryptor 625.    -   It provides encryption, decryption, and authentication services        to applications running on DHCT 333.

DHCTSE 627 includes a microprocessor (capable of performing DES),specialized hardware for performing RSA encryption and decryption, andsecure memory elements. All of the components of DHCTSE 627 arecontained in a single tamper-proof package, such as a package that uponattempting to access the information contained within the information isdestroyed. Only the components of DHCTSE 627 have access to theinformation stored in the secure memory elements. Any attempt by a userto gain access to any of the parts of DHCTSE 627 renders DHCTSE 627unusable and its contents unreadable. DHCTSE 627 may be an integral partof DHCT 333 or it may be contained in a user-installable module such asa “smart card”. The user “personalizes” the DHCT 333 by installing themodule in it.

FIG. 12 provides an overview of the components of DHCTSE 627. As shown,the components of DHCTSE 627 are all connected to a bus 1205. Beginningwith interface 1203 to the general purpose processor upon whichapplications execute in DHCT 333, interface 1203 permits passage of databetween the remaining components of DHCT 333 and DHCTSE 627, but doesnot permit components in the remainder of DHCT 333 to address and readthe contents of secret values in memory in DHCTSE 627. Microprocessor1201 executes the code for doing encryption, decryption, andauthentication and interpreting EMMs and ECMs; RSA hardware 1217 isspecial hardware performing the calculations involved with RSAencryption and decryption. Memory 1207 contains the code executed bymicroprocessor 1201, the keys, and the entitlement information. In apreferred embodiment, there are two kinds of physical memory in memory1207: ROM 1219, which is read-only memory whose contents are fixed whenDHCTSE 627 is manufactured, and non-volatile memory (NVM) 1209, whichcan be read and written like normal random-access memory, but whichretains its current values when DHCTSE 627 is without power.Non-volatile memory 1209 is organized as a set of non-volatile storagecells (NVSCs) 1211(0 . . . n), as described in U.S. Pat. No. 5,742,677,Pinder, et al., Information Terminal Having Reconfigurable Memory, filed3 Apr. 1995.

As will be explained in greater detail below, code executing inmicroprocessor 1201 dynamically allocates NVSCs 1211 to entitlementagents. In the preferred embodiment, NVM 1209 is used for the storage ofinformation that can be rewritten by means of EMMs, and ROM 1219 is usedfor code that will not change during the life of DHCTSE 627.

FIG. 13 is a schematic overview of the contents of memory 1207 in DHCTSE627. The memory is divided into two main parts: read-only storage 1301,which contains code and other information that does not change as aresult of the interpretation of EMMs, and NVA storage 1303, which isnon-volatile storage that changes as a result of the interpretations ofEMMs. RO storage 1301 contains code 1305.

Code 1305 falls into four categories: code 1307 for the encryption,decryption, and authentication operations performed by DHCTSE 627, codefor interpreting EMMs 1313, code for interpreting ECMs 1321, and codefor handling other CA messages such as the FPM and the GBAM. Code 1307includes code 1308 for the MD5 one-way hash algorithm, the code 1309 forthe RSA public key algorithm, and the code 1311 for the 3DES algorithm.EMM code 1313 falls into three classes: code 1315 which interprets EMMsreceived from a conditional access authority, code 1317 which interpretsEMMs employed by the entitlement agents to configure the storageallocation they receive from the CAA, and code 1319 which interpretsEMMs containing MSKs and entitlements. Code 1315, 1317 and 1319 thusimplements EMM manager 407 in a preferred embodiment. The code forinterpreting ECMs 1321 decrypts the control word contained in the ECMand checks whether DHCT 333 is permitted to access the instance of theservice that the ECM accompanies; if so, the code provides the decryptedcontrol word to service decryption module 625. The code for other CAmessages 1323 deals with messages such as the FPM and GBAM.

NVA storage 1303 has two main components: administrative storage 1330and EA storage 1331. Administrative storage 1330 contains DHCT keys1325, CAA keys 1329, and CAA data 1330. Beginning with DHCT keys 1325,each DHCT 333 has two public-private key pairs. The public key of one ofthe pairs serves as the public key used to encrypt EMMs sent to DHCT333, and the private key is used in DHCT 333 to decrypt the messages;the private key of the other of the pairs is used to encrypt the sealeddigests of messages sent by DHCT 333, and the public key is used byother network elements to decrypt the sealed digests of messagesreceived from DHCT 333. The pairs of keys are installed in DHCTSE 627when DHCTSE 627 is manufactured.

In a preferred embodiment, the manufacturer of DHCT 333 maintains acertified database that has the serial number of each DHCT together withthe pair of public keys belonging to it. When a CAA or EA wishes tobegin sending EMMs to a DHCT 333, it sends a message to control suite607 with the serial number of the DHCT. Control suite 607 responds tothe request by requesting the public key for the DHCT from a databasemaintained by the manufacturer of DHCT 333. The database responds to themessage by sending control suite 607 certified copies of the public keysfor the DHCT. The manufacturer thus functions as the certificationauthority for the keys. Control suite 607 stores the public keys in adatabase of its own. For details on key certification, see Schneier,supra, pages 425-428. Getting the public keys for the DHCT from themanufacturer has two advantages: first, it solves the problem ofcertifying the keys; second, because the public keys come from themanufacturer and not from DHCT 333, there is no requirement inconditional access system 601 that DHCT 333 have a reverse path tocontrol suite 607.

CAA keys 1329 are public keys for the conditional access authority. In apreferred embodiment, CAA keys 1329 include three public keys for theconditional access authority. These keys are originally installed whenDHCTSE 627 is manufactured, but may be changed in response to EMMs, aswill be explained in more detail below. CAA data 1330 includesparameters used by the CAA in managing EA storage 1331, and maps whichmap NVSCs belonging to particular entitlement agents to 8-bit names andthereby permit the CAA and the entitlement agents to manipulate theNVSCs 1211 by name.

Entitlement agent 1331 has EA information 1331 for each entitlementagent from which DHCT 333 containing DHCTSE 627 can obtain services. TheCAA uses EMMs to allocate NVSCs 1211 for an entitlement agent and theentitlement agent then uses EMMs to set the contents of its entitlementagent information 1333.

FIG. 14 shows how NVSCs 1211 are organized into EA storage 1331 in apreferred embodiment. There are two kinds of NVSC's 1211: “skinny”NVSCs, as shown at 1405, and “fat” NVSCs, as shown at 1409. A fat NVSCis made up of a number of skinny NVSCs. The storage 1403, which containsthe three CAA public keys, also contains two pointers: one, 1402, to afree list 1407 of unallocated skinny NVSCs and the other, 1404, to anentitlement agent list 1406 of allocated fat NVSCs 1409. There is such afat NVSC 1409(i) for each entitlement agent from which DHCT 333 mayreceive services. Each of these NSVCs 1409(i) may also have a list 1411of NVSCs, which may be skinny NVSCs 1405, fat NVSCs 1409, or acombination of both. A given NVSC 1409(i) and its list of skinny NVSCsmake up EA information 1333(i) for an EA. The fat NVSC 1409 is an EAdescriptor. As shown at 1333(i), the skinny NVSCs 1411 containinformation for the services provided by the entitlement agent such asan MSK for a service, a bit map of entitlement information, andinformation needed for interactive services such as IPPV.

Control of NVA Storage 1303

In a preferred embodiment, allocation and de-allocation of the NVSCs1211 may be ultimately controlled by either the CAA or DHCTSE 627. Whenthe CAA controls allocation and de-allocation, the CAA, usuallyrepresenting the operator of DBDS 501, negotiates with each of theentitlement agents and agrees on an allocation of the various types ofNVSCs for that entitlement agent. EA administrative code 1317 checkswhen it is interpreting EMMs from an entitlement agent to ensure thatthe entitlement agent does not use more NVSCs of each type than thoseallocated to it.

When DHCTSE 627 controls NVA storage 1303, the operator of the CAAnegotiates with each of the service providers and agrees on theallocation of storage needed for the services provided. The CAA thensends an encrypted message to the entitlement agent. The encryptedmessage contains the allocation based on data types, and the entitlementagent prevents the service provider from asking for more resources thanwere negotiated. If DHCTSE 627 nevertheless receives requests forstorage area above what is available in NVA 1303, it indicates to theuser of DHCT 333 via the user interface that no more storage isavailable and requests the user to either remove some service providerresources or to rescind the request.

Details of Operations Specified by EMMs

In the following, examples of operations specified by EMMs will begiven, beginning with changing a CAA public key, continuing throughestablishing an EA in DHCTSE 627, and ending with providing entitlementinformation for broadcasts, events, and interactive services. In thepreferred embodiment, a single CAA controls the allocation of EA storage1331 to entitlement agents. In other embodiments, there may be more thanone CAA. There are two kinds of entitlement information: that forbroadcast services and that for interactive services. Storage forbroadcast entitlements is more permanent than that for interactiveentitlements.

The amount of memory 1207 in DHCTSE 627 is limited. The CAA manages thisscarce resource and allocates it to the entitlement agents from whichDHCT 333 receives services. Different EAs may have different amounts ofstorage area allocated, depending on their needs. Once an EA hasreceived an allocation from the CAA, the EA may configure the storagearea within limits defined by the CAA. Different EAs may have differentlimits and different types of limits. At one extreme, the CAA onlyrestricts the total number of NVSCs 1211 that an EA may have in its EAinformation 1333. The CAA may impose tighter restrictions by limitingthe types of NVSCs 1211 and/or the number of each type. In this way, theCAA can prevent the EA from offering specific kinds of services and canlimit the amount of such services offered, i.e., the amount of time thatsuch services are offered.

When a CAA allocates fat and skinny NVSCs 1211 for an EA, it gives eachallocated NVSC 1211 a “name”, i.e., each NVSC 1211 has an identifier,such as an 8-bit identifier, that the CAA associates with the EA forwhich it has allocated the NVSCs 1211. The CAA and the EA use the namefor the NVSC 1211 to refer to it in EMMs that manipulate the NVSC. AnNVSC's name need not have anything to do with its physical location inNVM 1209. Since the name space is 8-bits wide, the names are assignedusing a 256-bit map. If an entitlement agent has the name of an NVSC, itmay make the NVSC into any type of NVSC as long as the type is one thatis permitted for the EA and as long as the total number of NVSCs of thetype belonging to the EA does not exceed the limit set by the CAA thatauthorized the EA.

Once the CAA has allocated the EA storage area in the DHCTSE, it is upto the EA to configure the storage area. The first step is to loadcertain parameters such as a PIN into a descriptor for the EA. Thesecond step is to determine which types of NVSCs are to be used for theprotected services to be offered. The names allocated by the CAA arethen distributed among the various types of NVSCs. Lastly, each NVSC isloaded by sending the appropriate EMM.

Addressing EMMs

In the conditional access layer, EMMs are addressed to a specific DHCTSE627, indexed by CAA or EA. This indexing is taken care of in EMM header1113, which includes a unique identifier for the CAA or EA that is thesource of the EMM, and that therefore is associated with the private keyused to make the EMM's sealed digest. The EMM header also includes theserial number for DHCTSE 627. The DHCTSE 627 responds only to those EMMsthat include its serial number. When a CAA is the source of the EMM,there is also a value in the header indicating which of the CAA publickeys is the public key for the source of the message. Conditional accessmessages may be transported in other data protocols, which may includeother addressing mechanisms. DHCTSE 627 ignores EMMs that are addressedto a CAA or EA that is not “known” by DHCTSE 627 (i.e., EMMs for whichthere is no CAA corresponding to the CAAID or EA that corresponds to theEAID). As will be explained in more detail below, information aboutindividual entitlements is contained in NVSCs 1211 for the entitlements.Each of these NVSCs has a type, and an EA may change the type orcontents of an NVSC 1211 by sending an EMM that specifies the name ofthe NVSC 1211 to be altered. DHCTSE 627 will alter the NVSC 1211 asindicated in the EMM unless the entitlement agent does not have an NVSCwith that name or the change violates a constraint set by the CAA. Inthose cases, the EMM is ignored by DHCTSE 627. Conditional access system601 does not require that digital broadband delivery system 501 have areverse path, or, if one exists, that any bandwidth on the reverse pathbe available to the EMM conditional access function. Consequently, DHCT333 does not return any acknowledgment, confirmation, or error messagesin response to an EMM. Therefore, the CAA or EA that is the source of anEMM should track the allocations of NVSCs 1211 and send only EMMs thatrequest legal operations. In other embodiments, a reverse path may berequired, and for these embodiments, the reverse path can be used foracknowledgment or error messages.

Changing a CAA

As previously indicated, a CAA is represented in DHCTSE 627 by itspublic key. Three public keys for the CAA are installed in DHCTSE 627when it is manufactured. A need may occasionally arise to change the CAAof DHCTSE 627. One circumstance under which such a need would arisewould be if the private key for the CAA had been compromised; anotherwould be if a new entity has taken over the function of authorizingentitlement agents. That might happen, for example, as a consequence ofthe sale of all or part of a DBDS 501.

Any one of the public keys for a CAA can be replaced by means of asequence of two EMMs, the first of which has a sealed digest encryptedwith the private key corresponding to a first one of the other twopublic keys, and the second of which has a sealed digest encrypted withthe private key corresponding to the second one of the other two privatekeys. Each of the two EMMs contains an identifier, the CAAID for the newCAA, a key select value indicating which of the three CAA public keys isto be replaced, and the public key for the new CAA. After the first EMMis successfully authenticated by DHCTSE 627 by verifying the digitalsignature applied by the first CAA key, DHCTSE 627 computes a MD5 hashof the new CAA public key in this first EMM and stores it. After thesecond EMM is successfully authenticated by the DHCTSE by verifying thedigital signature applied by the second CAA key, the DHCTSE computes aMD5 hash of the new CAA public key included in this second EMM. Thissecond hash is compared with the first. If the hashes are identical, thenew CAA public key and CAAID are substituted for the public key andCAAID of the CAA specified by the key select value. A single CAA publickey must not be changed twice without one of the other two CAA publickeys being changed in between.

Dynamically Adding and Removing Entitlement agents in DHCTSE 627: FIG.15

When a CAA authorizes a DHCT 333 to receive services from an entitlementagent, it does so by sending a sequence of EMMs that create anentitlement agent descriptor EAD 1409 for the new entitlement agent.FIG. 15 shows a detailed view of an EAD 1409(i) as created by the CAAEMMs. Header 1502 is common to all NVSCs 1211. Cell status 1501indicates whether the NVSC 1211 is allocated. Cell type 1503 indicateswhat kind of data it contains; with an EAD 1409. Cell type 1503indicates that the cell is a “fat” NVSC. Cell name 1505 is the 8-bitname that the CAA gives the cell when it allocates it. The names areper-EA. That is, the EA information 1333 for an EA may include up to 255NVSCs. Next element 1507 is a pointer to the next element in the list towhich the NVSC belongs. Thus, in an unallocated NVSC, it is a pointer tothe next NVSC in free list 1407; in an EAD 1409, it is a pointer to thenext element in EAD list 1406, and in a skinny NVSC that is part of alist 1411, it is the next skinny NVSC in that list. Next element 1507 isset in response to whatever EMM causes the list to be manipulated.

The remaining fields are particular to EADs 1409. The fields labeled1506 in FIG. 15 are all set by EMMs from the CAA. EAID 1509 is anidentifier for the entitlement agent to which EAD 1409 belongs; in thepreferred embodiment, EAID 1509 is used to locate EAD 1409 for a givenentitlement agent. CAA flags 1511 are a set of flags that indicate (1)the classes of service to which the entitlement agent can grant accessand (2) whether the public key for the entitlement agent is installed inEAD 1409. First skinny NVSC 1513 is a pointer to skinny NVSC list 1411belonging to EA information 1333 to which EAD 1409 belongs. EA maximums1515 define the maximum amounts of services for the EA to which EAinformation 1333 belongs. The last field 1506 set by the CAA is EApublic key 1527, which is the public key for the EA to which EAinformation 1333 belongs.

The fields in EA fields 1516 contain information that is associated withthe customer to whom DHCT 333 belongs. The fields are set by an EMMreceived from the EA after EAD 1409 has been allocated and fields 1506have been set. DHCT flags 1517 include flags indicative of the servicesprovided by the EA that this specific DHCT 333 is presently entitled toreceive. Stored credit limit field 1519 is used with instances ofimpulse services, i.e., instances of services that need not be purchasedin advance. Stored credit limit field 1519 indicates the maximum amountof a service that an interactive customer can use without authorizationfrom the EA. As will be explained in detail below, authorization isobtained by sending an FPM to the EA and receiving a confirming EMM fromthe EA. X coordinate 1521 and Y coordinate 1523 define a location ofDHCT 333 in a coordinate system (to be explained more fully later)established by the entitlement agent. The coordinate system may begeographic and may, for example, be used to determine whether the DHCT333 is in an area that is to be blacked out in a broadcast. Thecoordinate system may also be used generally to define subsets of anEA's customers. For instance, the X coordinate and Y coordinate could beused to define customers who do not wish to receive movies that haveratings other than G or PG-13. The PIN is a multi-character code thatthe customer for the DHCT uses to identify himself or herself to theentitlement agent.

The EMMs that the CAA sends to set up EA information 1333 for an EA arethe following:

-   -   Set EA Allocation Name Map    -   Set EA Maximum Allocations    -   Update Entitlement Agent Public Key

EMM header 1113 in all of these EMMs contains a CAAID for the CAA, andall of the EMMs have a sealed digest that has been encrypted with theCAA's private key. The CAA may use these EMMs not only to set up EAinformation 1333, but also to modify already existing EA information1333 for an EA and to remove EA information 1333 for an EA. When thelatter has been done, DHCTSE 627 will no longer respond to EMMs or ECMsfrom the entitlement agent.

Set EA Allocation Name Map

The Set EA Allocation Name Map EMM contains an EAID, which uniquelyidentifies the EA for which the EA information 1333 is being created ormodified, and a name map. The map has a bit for each name; when the CAAhas allocated a NVSC for the EA, the bit corresponding to the NVSC'sname is set. CAA EMM code 1315 responds to this EMM by allocating theNVSCs required for EA information 1333, mapping the names for the EAIDto the physical locations of NVSCs, making list 1411 and setting firstNVSC flag 1513 to point to it, adding the new EA Descriptor 1409 to thehead of EA list 1406 and setting next element pointer 1507 accordingly,and filling out header fields 1502 and EAID field 1509.

CAA EMM code 1315 stores the current name map for the EA in CAA data1330 and consequently can compare the name map in a newly-received SetEA Allocation Name Map EMM with the current name map. If a name isspecified in both name maps, the Set EA Allocation Name Map command doesnot affect the NVSC 1211 with the name. If the name map in the EMMspecifies a name that was not in the current name map, an NVSC 1211corresponding to that name is added to list 1411. If the name map in theEMM no longer specifies a name that was previously allocated to theentitlement agent, the NVSC 1211 corresponding to that name is returnedto free list 1407. After this is done, the name map in the EMM becomesthe current name map.

Typically, an entitlement agent and a conditional access authority willcooperate in determining how large list 1411 should be. For example, ifan entitlement agent needs less space, it will send a message to thateffect to the CAA, the message will contain the names of the NVSCs 1211that the entitlement agent wishes to have removed, and the name map inthe EMM sent by the CAA will specify only the names of the NVSCs 1211that the entitlement agent wishes to keep. It may, however, happen thatthe entitlement agent is not cooperative or that the conditional accessauthority must reduce the size of list 1411 for the entitlement agentbefore it receives a message from the entitlement agent. In that case,the CAA may remove NVSCs 1211 from list 1411 by the value of the name,beginning with the name with the highest numeric value, continuing withthe next highest, and so on, until the required number of NVSCs 1211have been removed.

The CAA can also use the Set EA Allocation Name Map EMM to remove EAinformation for an EA from DHCTSE 627. When the EMM is used in thisfashion, none of the bits in the name map are set. CAA EMM code 1315responds by returning all of the NVSCs in the EA information 1333 and EADescriptor 1409(i) for the EA identified by the EAID in the EMM to freelist 1407 and re-linking EA list 1406 as required.

Set EA Maximum Allocations

The Set EA Maximum Allocations EMM contains the EAID for the EA havingthe entitlement information 1333 that is being created or modified andalso contains values for fields 1511 and 1515 of EAD 1409. CAA EMM code1315 responds to this EMM by reading down EA list 1406 until it finds EAdescriptor 1409 with the EAID specified in the EMM and then settingfields 1511 and 1515 of EAD 1409 using the values in the EMM. When anentitlement agent sends an EMM to DHCTSE 627 that establishesentitlement information of a certain type, for example, for an event,the code that interprets the EMM checks the EA maximum allocations todetermine whether the maximum number of entitlements for that EA hasbeen exceeded. In the preferred embodiment, entitlements are representedby NVSCs. Consequently, what is limited is the number of NVSCs of agiven type in list 1411.

Update Entitlement agent Public Key

The Update Entitlement Agent Public Key EMM contains the EAID for the EAhaving the entitlement information that is being created or modified andthe EA's public key. CAA EMM code 1315 responds to this EMM by locatingEA descriptor 1409 as described above and setting field 1527 from thepublic key in the EMM. With the EA's public key in place, DHCTSE 627 canthen use the signed digests of the EMMs to verify that they are from theEA. This verification is possible since the EA uses the private keycorresponding to the updated public key to perform the signingoperation.

EA EMMs that Modify Entitlement Information 1333

The EA EMMs that modify entitlement information have sealed digests thatare encrypted using the EA's private key. The EMMs fall into two groups:EMMs that modify EA fields 1516 of EAD 1409 and EMMs that modifycontents of the NVSCs making up list 1411. As set forth with regard toEAD 1409, each NVSC has a name, and each NVSC in list 1411 has a type.An NVSC is named by the CAA, as described above, and its name cannot bechanged by the entitlement agent. The entitlement agent can, however,change the type and contents of a NVSC, subject only to the maximums forthe types established in EAD 1409 for the EA. It is up to theentitlement agent to keep track of the types and contents of the NVSCsin EA information 1333.

The EMM that modifies EA fields 1516 of EAD 1409 is the UpdateEntitlement Agent Properties EMM. The second group of EMMs is furthersubdivided according to the kinds of entitlements they provide. Thereare two broad families of entitlements: broadcast entitlements fornon-interactive services and interactive entitlements for interactivesessions. Within the broadcast entitlements, there are further evententitlements for events that the user pays for individually, as is thecase with pay-per-view events, interactive pay-per-view events, and nearvideo-on-demand events. The non-event broadcast EMMs include:

-   -   Update MSK    -   Update Digital Bit Map    -   Update Digital List    -   Update Analog MSK and Bit Map    -   Update Analog MSK and List    -   Update Analog Bit Map    -   Update Analog List

The broadcast EMMs for events include:

-   -   New Event Storage    -   Add/Remove PPV Event    -   Acknowledge IPPV/NVOD Event

The EMMs for interactive sessions include:

-   -   New Interactive Session Storage    -   Add Interactive Session    -   Remove Interactive Session

As can be seen from the names of the EMMs, the EA can change the type ofthe named NVSCs allocated by the CAA as needed for events andinteractive sessions, subject only to the maximums specified in EAD1409.

There are separate CAA EMMs for allocating NVSCs, setting limits ontypes of NVSCs, and assigning a public key to an entitlement agent.Also, the EA EMMs for writing NVSCs 1211 do so by name and can changethe NVSC 1211 type as well as its content. Therefore, access controlsystem 601 has a high degree of control and flexibility. A CAA maydynamically constrain the total number of entitlements that anentitlement agent may give, the types of entitlements, and the number ofentitlements of each kind as required. The CAA may also change theconstraints either in part or as a whole, and can do so either incooperation with the entitlement agent or unilaterally. Within theconstraints imposed by the CAA, however, the entitlement agent is freeto dynamically manage its own entitlements, changing not onlyentitlements of a given type, but even changing the types themselves.

Update Entitlement Agent Properties

This EMM contains the values for EA fields 1516 of EAD 1409. EAadministration EMM code 1317 reads EMM header 1113 to get the EAID forthe EA to which the EMM is directed and simply sets fields 1516 in EAD1409 for the EA from the EMM.

Non-Event Broadcast EMMs

Of the non-event broadcast EMMs, four types will be discussed here.These are Update MSK, Update Bit Map, Update List, and updatecombinations with MSK and list or bitmap. Those skilled in the art willbe able to easily apply the principles explained below to EMMs thatperform the functions indicated by the names of the other non-eventbroadcast EMMs. For example, the principles of digital EMMs can beapplied to analog EMMs. There is a separate type of NVSC 1405 for eachinformation type provided by the above non-event broadcast EMMs. FIG. 16shows the contents of four of these types of NVSCs. Each NVSC type willbe discussed together with the EMM that provides the information itcontains.

Update MSK

The Update MSK EMM is used to send a new MSK for a set of servicesprovided by the EA specified by the EMM. The new MSK and otherinformation associated with the MSK are stored in MSK NVSC 1601 in list1411 for EA information 1333 belonging to the EA specified by the EMM.Included in MSK NVSC 1601 is header 1502. Header 1502 specifies thatNVSC 1601 is a MSK NVSC, gives the NVSC's name, and contains nextelement pointer 1507 to the next element in list 1411. The other fieldscontain information about the MSK. In the preferred embodiment, MSK 1608has two 128-bit parts: the even MSK 1609 and the odd MSK 1611. Each parthas two halves, i.e., a first half and second half, each of which has 56key bits and 8 unused parity bits. The MSK 1608 is associated with apair identifier 1603 for MSK 1608, an expiration date 1605 for MSK 1608,and a flag 1607 indicating whether the value of expiration date 1605should be ignored. If the expiration date 1605 is not to be ignored,DHCTSE 627 will not use MSK 1608 to decrypt a control word after theexpiration date. The identifier 1603 is per-EA, and consequently, agiven EA may have one or more MSK NVSCs 1601 at any given time to storea plurality of different MSKs. Thus, conditional access system 601 notonly permits separate security partitions for each EA, but also permitssecurity partitions within an EA.

The Update MSK EMM header contains the EAID needed to locate EAinformation 1333 for the EA; the message contains the name of the NVSCthat is to receive the MSK, a MSK pair selector which specifies a MSKpair ID for the MSK to be updated, a set of flags permitting the EA toselectively change MSK pair ID 1603, expiration date 1605, no expirationdate 1607 and either half of MSK 1608, and the information needed tomake the changes. At a maximum, the EMM contains a value for MSK pair ID1603, a value for expiration date 1605, a value for no expiration date1607, and values for even MSK 1609 and odd MSK 1611. EA MSK code 1319processes the Update MSK EMM by locating EA Information 1333 for the EAidentified by the EMM header's EAID, using the cell name to locate theproper NVSC, giving that NVSC the MSK type, and then writing to the MSKNVSC 1601 as required by the flags and the information in the EMM. Thisprocedure is the same for both analog and digital Update MSK EMMs. Thedifferences are in the EMM command code in EMM Header 1123 and NVSC type1503.

Entitlement Identifiers

As will be explained in more detail below, an ECM specifies the serviceinstance that it accompanies by means of (1) the EAID for theentitlement agent that is the source of the ECM and (2) a 32-bitentitlement ID for the instance. Entitlement IDs are per-EA. By makingthe entitlement IDs 32 bits long, each EA will have enough entitlementIDs even for transient services such as pay-per-view events andinteractive services. In the preferred embodiment, when DHCTSE 627interprets an ECM, it checks whether DHCT 333 is entitled to decrypt theinstance by looking in EA information 1333 for the EA specified in theECM for an entitlement ID that corresponds to the entitlement IDspecified in the ECM. The entitlement IDs in the EMM and in EAinformation 1333 can be represented in at least two ways. One way is bysimply listing entitlement IDs. The drawback with this technique is thatthe 32-bit entitlement IDs are large, and NVSCs are a scarce resource.The other way is by means of a starting entitlement ID value and a bitmap. Any entitlement ID having a value within 255 of the entitlement IDvalue specified by the starting entitlement ID value can be specified bysetting a bit in the bit map. This technique is set forth in the Bankerand Akins patent application supra. See particularly FIG. 2 of theBanker and Akins patent application and the discussion of that figure.The following discussion of specifying entitlement IDs by means of astarting ID and a bit map is an expansion of the discussion in thatpatent application.

Update Bit Map EMM

This EMM updates a bit map that specifies one or more entitlement IDs.The bit map is stored in an entitlement bit map NVSC 1613. NVSC 1613 hasa header 1502 with the cell number and type of the NVSC; a firstentitlement ID 1615, which is the first entitlement ID which may bespecified by the bit map; an expiration date 1617, which specifies whenthe entitlement IDs specified by first entitlement ID 1615 and the bitmap expire; a no expiration date flag 1619, which indicates whetherthere is in fact an expiration date; and bit map 1621. The update bitmapEMM contains the cell name for the NVSC 1613 to be set, a set of flagsthat indicate the information in NVSC 1613 that is to be set by the EMM,and the values for the information. The EMM may set any or all of firstentitlement ID 1615, expiration date 1617, no expiration date 1619, andbit map 1621. EA administrative EMM code 1317 responds to the EMM bysetting the fields of the specified NVSC 1613 as indicated in the EMM.This procedure is the same for both Update Digital Bit Map and UpdateAnalog Bit Map EMMs. The differences are in the EMM command code in EMMHeader 1123 and NVSC type 1503.

Update List EMM

The Update List EMM updates a list of entitlement IDs that is containedin an entitlement list NVSC 1623. NVSC 1623 has a header 1502 with thecell name and type for the NVSC and contains up to six entitlement IDelements 1625. Each of the elements contains an entitlement ID 1627, anexpiration date 1629 for the entitlement ID, and a flag 1631 indicatingwhether the entitlement ID has an expiration date. The update list EMMcontains the cell name for the NVSC, a value for the flag, an expirationdate, and values for up to six entitlement ID elements 1625. Thisprocedure is the same for both Update Digital List and Update AnalogList EMMs. The differences are in the EMM command code in EMM Header1123 and NVSC type 1503.

Broadcast Events

A broadcast event is a one-time service, such as a pay-per-viewbroadcast of a boxing match. In the preferred embodiment, there are twokinds of broadcast events: ordinary pay-per-view broadcast events, inwhich the customer has ordered in advance to see the event, and impulseevents where the customer decides at the time the event is broadcastthat he wants to order it. There are different kinds of impulse events,such as: impulse pay-per-view (IPPV) events, which are pay-per-viewevents where the customer can decide at the time of the event topurchase it, and near video-on-demand (NVOD), where popular movies arerebroadcast at short intervals and the customer can decide when therebroadcast occurs whether he or she wants to view it. Those skilled inthe art will realize that the concept of an “event” can refer to anyservice over a specific time period (whether broadcast ornon-broadcast), such as video on demand events or other types of eventsnot listed here.

In the case of pay-per-view events, the customer orders the event fromthe entitlement agent, and the agent responds by sending an EMM thatcontains the necessary entitlement information. In the case of eventswhere the customer decides at broadcast time that he or she wants topurchase the event, purchase information, i.e., information about theentitlements that can be purchased, must be distributed with the event.In these cases, the purchase information is distributed by means ofglobal broadcast authenticated messages, or GBAMs. The customer providesinput 628 that specifies a purchase. The DHCT 333 responds to the input628 by storing the record of purchase in the DHCTSE 627 and thenbeginning to decrypt the event. Later, the DHCT 333 sends theentitlement agent a forwarded purchase message (FPM) indicating what hasbeen purchased by the customer, and the entitlement authority respondswith an EMM that confirms the purchase and contains the necessaryentitlement information. The record of the purchase remains until an EMMconfirming the purchase is received by the DHCTSE 627.

Event NVSCs: FIG. 17

FIG. 17 shows event NVSC 1701 used to store entitlement information forevents. Header field 1502 is similar to that for other NVSCs 1701. Eachevent NVSC 1702 may contain up to three event descriptors 1703, each ofwhich describes a single event. Each event descriptor 1703 contains aFlags Field 1705 that includes flags to indicate (1) whether the eventis active, (2) whether its end time has been extended, (3) whether theentitlement agent has confirmed purchase of the event, (4) whether thecustomer can cancel at any time, (5) whether the customer can cancel ina cancellation window, (6) whether the customer has canceled thepurchase, (7) whether the right to copy the event has been purchased,and (8) whether the event is an analog or digital service. Purchase time1709 is the later of the start time for the event or the time thecustomer purchased the event. End time 1709 is the time the event is toend. Cost 1711 is the cost of the event to the customer, and entitlementID 1713 is the entitlement ID for the event.

New Event Storage EMM

When the CAA sets up entitlement agent descriptor 1409 for anentitlement agent, it includes a value in EA Maximums 1515 that limitsthe number of event NVSCs 1701 the entitlement agent may have. Withinthat number, however, the entitlement agent is free to allocate eventNVSCs 1701 from the total number of NVSCs 1405 belonging to theentitlement agent and to reuse existing event NVSCs 1701. To allocate anevent NVSC, the EA uses the new event storage EMM, which simply containsthe cell name for the NVSC that is to be allocated. Once the event NVSC1701 has been allocated, its fields are set as follows:

-   -   In the case of an ordinary PPV event, fields are set by an        add/delete event EMM;    -   In the case of an IPPV or NVOD event, fields are set in part        from the GBAM for the event and in part from customer input 628.

The contents of an event NVSC 1701 are deleted by an add/delete eventEMM or by receiving an ECM containing a time greater than the event endtime in the event NVSC 1701, if the event record had been previouslyacknowledged by receiving the Acknowledge Event EMM.

The Add/delete Event EMM

The add/delete event EMM contains a flag that indicates whether the EMMis setting or deleting an event. In the latter case, the contents of theEMM must match the current contents of the NVSC 1701 that is to bedeleted. In the former case, the values of the EMM include flagsindicating whether time extensions are allowed and whether the right tocopy has been purchased. Further included are values for the event'sstart time and end time and the entitlement ID for the event. When theadd/delete flag indicates “delete”, EA administrative code deletes thecontents of the NVSC 1701. When it indicates “add”, the code sets thecorresponding fields of the NVSC 1701 to the values specified in theEMM. The flag that indicates whether the EA has acknowledged thepurchase is set to so indicate.

The Global Broadcast Authenticated Message: FIGS. 18-20

The Global Broadcast Authenticated Message (GBAM) is, like the EMMs,ECMs, and FPMs, a CA message. A GBAM is broadcast by an entitlementagent to DHCTs 333. FIG. 18 shows a CA message 805 including a GBAM1801. Message 805 includes a CA message header 1003 and a CA GBAMmessage 1803, which in turn is made up of a GBAM header 1807 and globalbroadcast data 1809. Global broadcast data 1809 is not encrypted, butGBAM 1801 is authenticated in the same fashion as an ECM: header 1807,global broadcast data 1809, and MSK 1015 belonging to the EA which sentthe GBAM are hashed by one-way hash function MD5 to produce GBAM MAC1805. As with the ECM, the MSK 1015 is a shared secret between the EAthat sent the GBAM and DHCTs 333 that have EA information 1333 for theEA.

FIG. 19 shows GBAM header 1807 in detail as well as the form that globalbroadcast data 1809 takes when GBAM 1801 is used to provide entitlementinformation for IPPV or NVOD. GBAM header 1807 has a conditional accesssystem ID 1901 that identifies CA system 601 in which GBAM 1801 is beingused, a tag that indicates that the message is a GBAM, and theidentifier 1905 of the entitlement agent sending the GBAM. Fields 1907and 1909 specify the key that was used to make MAC 1805. Field 1907specifies the parity of the MSK half used to make the digest, and MSKselect 1911 is an identifier for the MSK itself.

Purchasable entitlement data 1913 refers to the form of global broadcastdata 1809 that is used to provide entitlement information for IPPV orNVOD. Of the fields that are relevant for the present discussion,Entitlement ID 1915 is the entitlement ID for the event associated withthe GBAM, and Flags 1917 include flags indicating what kind ofcancellation is allowed and whether the time for the event may beextended. Number of modes 1919 indicates how many different modes thereare for purchasing the event. The rights that the purchaser receives tothe event and the price the purchaser must pay will vary with the mode.In the preferred embodiment, an event may have up to five purchasemodes. If more purchase modes are required, additional GBAMs may besent. The rights and prices for each mode are indicated by arrays. Eacharray has as many valid elements as there are modes. The value of anelement corresponding to a mode indicates the right or price for thatmode. Thus, mode right to copy field 1921 is a bit array; if a bit for amode is set, the purchaser of the mode has the right to copy the event.Similarly, mode length field 1927 contains a value for each mode thatindicates the length of time for the event in that mode. Mode cost field1929 contains a value for each mode that indicates the cost for theevent in that mode. Earliest start field 1923 gives the earliest time atwhich entitlement for the event can start, and latest end field 1925gives the latest time at which entitlement must end.

When DHCT 333 receives GBAM 1801, it passes GBAM 1801 to DHCTSE 627 forauthentication of global broadcast data 1809. Authentication will failunless DHCTSE 627 has the required MSK. If (1) DHCTSE 627 has therequired MSK and (2) global broadcast data 1809 is data 1913, DHCT 333permits the customer to purchase the event. In so doing the customeridentifies himself or herself to DHCT 333 by means of a PIN, and thatPIN must match PIN 1525 in EAD 1409 for the entitlement agent that sentthe GBAM. In making his or her purchase, the customer also specifies therelevant modes. Given the mode information and the cost information inthe GBAM, DHCT 333 can determine whether ordering the impulse event willcause the customer to exceed the amount (of time, money, etc.) specifiedin stored credit limit 1519 in EAD 1409. If the customer has notexceeded the limit, the information from the GBAM and from thepurchaser's inputs are used to make an event descriptor 1703 for theevent. DHCT 333 passes the information to DHCTSE 627, which sets thefields in event descriptor 1703 according to the values provided it byDHCT 333. The flag that indicates whether the purchase information hasbeen acknowledged is cleared, and the cost of the event is added to thecurrent credit balance.

The Forwarded Purchase Message: FIG. 21

The forwarded purchase message (FPM) in a preferred embodiment servestwo purposes:

-   -   it informs the entitlement agent that the customer has purchased        an IPPV or NVOD event; and    -   it informs the entitlement agent that the customer has canceled        the purchase of any event.

In other embodiments, messages like the FPM can be used to transfer anykind of information from DHCT 333 to a CAA or an EA. For example, such amessage can be used to transfer monthly order information from DHCT 333to an EA.

DHCT 333 sends a forwarded purchase message with the purchaseinformation via the reverse channel to the entitlement agent that sentthe GBAM. The FPM is contained in a reverse channel data packet that isaddressed to the EA. FIG. 21 provides an overview of the FPM and of thecryptographic measures used to protect its contents. FPM 2101 is a CAmessage 805 and consequently is sent with a CA message header 1003. FPM2101 itself is made up of FPM encrypted envelope key 2103, whichcontains the EAID for the entitlement agent and FPM key 2119 fordecrypting the purchasing information contained in FPM encrypted events2113. The key and other contents of envelope key 2103 are encrypted forprivacy using the public key of the entitlement agent for which FPM 2101is intended. CA FPM message 2105 includes CA FPM header 2111, whichincludes the EAID for the intended EA, and FPM encrypted events 2113.The latter are encrypted using the 3-DES algorithm with the key inenvelope key 2103. CA FPM message 2105's parts are a header 2131, FPMclear events 2133, which contains the purchase information, and padding2135. The last part of FPM 2101 is FPM signed authentication 2107, whichis encrypted with the private key of DHCT 333 from which FPM message2101 is sent. The encrypted material includes FPM signing header 2125,FPM MAC 2127, and padding 2129. FPM MAC 2127 is made using the MD 5one-way hash algorithm from FPM clear events 2133. Only the EA for whichthe FPM is intended can decrypt envelope key 2103 to obtain key 2119 todecrypt FPM encrypted events 2123, and the EA can check the authenticityof FPM clear events 2133 only if it has the public key for DHCT 333 fromwhich FPM 2101 was sent.

The part of FPM 2101 that is of further interest here is FPM clearevents 2133. The information in that part of the FPM includes the serialnumber of DHCTSE 627 in DHCT 333 from which the message came, the EAIDof the destination EA, and an indication of the number of events forwhich the FPM contains purchase information. The information for eachevent is contained in forwarded event data for that event. The forwardedevent data is taken from GBAM 1801 and event descriptor 1703 for theevent. Fields of interest in the present context include flagsindicating (1) whether the event has been extended, (2) whether the userhas canceled the event, and (3) whether the customer has purchased theright to copy. Other information includes the time the event started orwas purchased, whichever is later, the time the event is to end, itscost to the customer, and the entitlement ID for the event. To cancelany event, including an ordinary pay-per-view event, DHCT 333 sends anFPM with the same message, but with the event canceled flag set toindicate cancellation. The conditions under which DHCT 333 sends an FPMcancellation message will be explained in more detail below. FPMs mayalso be used to purchase other service types, such as monthlysubscriptions, or data downloads, for example.

The Acknowledge IPPV/NVOD Event EMM

When the entitlement agent receives the FPM, it enters the informationcontained in the FPM in its customer information database and returns anacknowledge IPPV/NVOD event EMM to DHCT 333. EMM command data 1125 inthis EMM contains an exact copy of the forwarded event data in the FPMthat the EMM is acknowledging. When DHCTSE 627 receives this EMM, itdecrypts and authenticates it and then, for each item of copiedforwarded event data, it uses the entitlement ID to locate event NVSC1701 for the event. Having located the event NVSC 1701, it compares thecopied forwarded event data with the corresponding fields of event NVSC1701. If they are the same, DHCTSE 627 sets the flag in Flags Field 1705that indicates that the purchase has been confirmed and adjusts thestored credit balance. If the EMM has its “canceled” flag set, the “inuse” flag in event NVSC 1701 is set to indicate that event NVSC 1701 isnot in use and is therefore available for reuse by the entitlementagent.

Other Uses of GBAM 1801

GBAM 1801 can be used generally to broadcast authenticated messages viaa MPEG-2 transport stream, or other transport mechanisms, to DHCTs 333.CA system 601 itself uses GBAM 1801 in two other ways: to periodicallybroadcast a time value to DHCTs 333 and to extend the time for events.In the former case, GBAM 1801 simply carries the time value, which is asecure time, due to the GBAM's authentication. The code in DHCT 333 thatcarries out a task for the entitlement agent that sent the system timeGBAM can use the time value to coordinate its activities with activitiesby the EA. Note that this arrangement permits the use of per-entitlementagent time schemes. It also permits establishing a uniform system timethroughout a digital broadband delivery system by setting up oneentitlement agent in each DHCT 333 of the digital broadband deliverysystem as the “system time entitlement agent” and addressing the systemtime GBAM to the system time entitlement agent.

GBAMs 1801 that extend the time for an event carry the entitlement IDfor the event and the number of minutes the time for the event is to beextended. When GBAM 1801 is received and provided to DHCTSE 627, thesecure element adds the number of minutes to end time 1709.

FIG. 20 shows a server application 2001 executing on a processor havingaccess to entitlement agent 2005 and to the MPEG-2 transport streambeing received by a group of DHCTs 333. The server application 2001 canuse GBAM 1801 to send authenticated messages to the DHCTs 333. Serverapplication 2001 sends a message to entitlement agent 2005, which usesits transaction encryption device 603 to make a GBAM 1801 including thepayload. Entitlement agent 2005 then returns the GBAM to serverapplication 2001 that sends application data together with the GBAM, asshown at 2007, to client application 2009 in the DHCTs 333. Each clientapplication sends GBAM 1801 to DHCTSE 627, which authenticates it. Ifthe authentication succeeds, DHCTSE 627 sends an acknowledgment toclient application 2009. It should be noted here that it is theentitlement agent and not server application 2001 that authenticates thepayload.

NVSCs and EMMs for Interactive Sessions

DBDS 501 can also be used for interactive sessions. Examples of suchuses are browsing the Internet or playing video games. In suchapplications, data being sent to the customer will generally go via theMPEG-2 transport stream, while data being sent from the customer will govia the reverse channel. Such an arrangement is advantageous for themany interactive applications in which the customer receives a largeamount of data, for example, the data that represents an image, makes ashort response, and then receives another large amount of data.

Each interactive session that is currently taking place with a user ofDHCT 333 has an interactive session NVSC 1211 in list 1411 belonging tothe entitlement agent that grants access to the interactive session. Theinteractive session NVSC contains a session key for the interactivesession and an entitlement ID for the interactive session. DHCTSE 627allocates the interactive session NVSC in response to a new interactivesession storage EMM from the entitlement agent. The new interactivesession storage EMM simply contains the cell name of the NVSC to be usedfor the interactive session.

Once the EA has established the NVSC, it sends an “add interactivesession” EMM that is directed to the name of the newly-allocated NVSCand contains the entitlement ID and the key for the interactive session.The secure element places the entitlement ID and key in the NVSC. Whenthe EA determines that the interactive session is over, it sends a“remove interactive session” EMM with the entitlement ID for theinteractive session and the secure element deletes the contents of theNVSC. It is of course possible that the entitlement agent sends a newinteractive storage EMM at a time when all of the interactive sessionNVSCs allotted by the CAA to the EA are already in use. DHCTSE 627 in apreferred embodiment deals with this situation by keeping track of thelast time each interactive session sent or received data. When a newinteractive session is needed and none is available, DHCTSE 627 shutsdown the interactive session that least recently sent or received dataand uses that interactive session's interactive session NVSC for the newinteractive session. Another solution is to request the user to selectan interactive session to be terminated.

Details of the ECM: FIG. 22

The information in an ECM that is used to determine whether the instanceof a service that the ECM accompanies is to be decrypted in a given DHCT333 is contained in ECM entitlement unit message 1011. FIG. 22 givesdetails of the contents of ECM entitlement unit message 1011 for apreferred embodiment of the present invention. Beginning with message ID2205, the two fields 2201 and 2203 identify this message as an ECMentitlement unit message. EAID 2207 is the identifier for theentitlement agent that grants entitlements to access to the instance ofthe service that the ECM accompanies.

Decryption information 2209 is information used to produce the controlword 2235. Control word counter value 2235 is encrypted using the 3DESalgorithm in a preferred embodiment. This algorithm employs two keys,and in a preferred embodiment, each key is ½ of the MSK. Also, there aretwo versions of the MSK: even and odd. MSK parity 2211 specifies whichversion is to be used in the 3DES algorithm. MSK ID 2213 specifies whichMSK belonging to the entitlement agent is to be used, or if the ECMaccompanies data for an interactive session, it specifies that the keyis to be found in the NVSC for the interactive session. Control wordparity 2215 specifies the parity of the unencrypted control word 2235.Parity count 2217 is a 0-1 counter that has the value 0 when the parityof the control word is even and 1 when it is odd.

Free preview 2219 is a flag that indicates that the ECM is accompanyinga portion of the service instance that is a free preview. That is, aslong as a customer has the MSK for decrypting the service instance, thecustomer needs no further entitlements to view the free preview portionof the service. The main use of free previews is with IPPV or NVODservices. Copy protection level 2221 is a value that indicates to whatextent the instance may be copied. Blackout/spotlight 2223 is a valuethat indicates how blackout/spotlight information 2236 is to be used:not at all, for a blackout, or for a spotlight (i.e., the service istargeted to the specific area).

Number of entitlement IDs 2225 specifies the number of entitlement IDs2245 that are contained in this ECM. The maximum number in a preferredembodiment is six in a single ECM. Multiple ECMs may be sent for eachservice. Allow IPPV 2229 is a flag that indicates whether the serviceinstance may be viewed on an IPPV or NVOD basis. Cancel window 2231 is abit that is set in a service instance that may be viewed as an event toindicate the end of the period during which the customer may cancel theevent. Time stamp 2233 is a time stamp indicating the time at which theECM was created. Encrypted control word 2235 is the control wordcontained in the ECM. It is encrypted using the 3DES algorithm and theMSK for the service instance.

Blackout/spotlight information 2236 defines a geographic area which isto be blacked out or spotlighted by an instance of a service. It does soby means of x centroid 2239 and y centroid 2241, the two of which definea point in a geographical coordinate system defined by the entitlementagent, and blackout radius 2237, which is used to determine a squarethat is centered on the point defined by fields 2239 and 2241 and thathas sides that are twice the value of blackout radius 2237. EntitlementID list 2243 contains from one to six entitlement IDs for the instanceof the service that the ECM accompanies.

Details of Blackout/Spotlight Info 2236: FIGS. 26 and 27

The coordinate system used in a preferred embodiment is shown in FIG.26. Coordinate system 2601 is a 256 unit by 256 unit square, with theorigin at the lower left-hand corner. In the coordinate system, it isthe lines, rather than the spaces between them, that are numbered. Theentitlement agent to which coordinate system 2601 belongs assigns eachDHCT 333 in the area covered by the coordinate system the coordinates ofan intersection of a line that is perpendicular to the x axis with aline that is perpendicular to they axis. Thus, a DHCT 333(k) may beassigned the point (i,j) 2603 in coordinate system 2601.

FIG. 27 shows how areas are defined in coordinate system 2601. Area 2705has its centroid 2701 at the point whose coordinates are (57,90). Theradius 2703 of the area is three, so this number is added to andsubtracted from each of the coordinates of the centroid to produce asquare 2705 whose lower left-hand corner is at (54,87) and whose upperright-hand corner is at (60,93). In the preferred embodiment, points onthe left and bottom lines are in the area; points on the top and rightlines are not.

Determining Whether to Decrypt the Service Instance that Accompanies anECM

Conceptually, what happens when DHCT 333 receives an ECM accompanying aninstance of a service is that DHCT 333 provides the ECM to DHCTSE 627,which examines the NVSCs in EA storage 1331 to find whether the customerto whom DHCT 333 belongs is entitled to receive the instance of theservice. If the customer is so entitled, DHCTSE 627 decrypts the controlword in the ECM and provides it to service decryptor 625, which uses itto decrypt the MPEG-2 packets containing the audio and video for theservice. However, the number of different kinds of services, the numberof different ways in which a service can be purchased, and the number ofways in which access can be restricted all work together to make themanner in which DHCTSE 627 processes an ECM rather complex.

The simplest case is for a broadcast service such as a standard CATVchannel. Here, the customer who owns DHCT 333 has paid his or hermonthly bill for the service and the entitlement authority has sent twoEMMs to DHCT 333: a MSK EMM with the month's MSK for the service and anEMM that specifies the entitlement ID for the service. As previouslypointed out, the latter EMM may either contain a list of entitlement IDsor a first entitlement ID and a bit map. All of these EMMs may alsocontain expiration dates: in the case of the MSK EMM, there is anexpiration date of the MSK; in the case of the entitlement ID list EMM,there is an expiration date for each entitlement ID on the list; in thecase of the entitlement bit map EMM, there is an expiration date for theentire bit map.

At a minimum, EA information 1333 for the entitlement agent thatprovides entitlements for the service instance that the ECM isaccompanying contains EA descriptor 1409, a MSK NVSC 1601, and either anentitlement bit map NVSC 1613 or an entitlement list NVSC 1623 for theservice to which the instance belongs. EA information 1333 may alsocontain NVSCs with entitlement information for many other services orinstances thereof. The ECM for the service instance will contain, at aminimum, entitlement agent ID 2207, decryption information 2209, timestamp 2233, encrypted control word 2235, and a single entitlement ID2245 for the instance of the service.

When DHCT 333 receives the ECM, it delivers the ECM to DHCTSE 627, whichreads down EA list 1406 until it finds an EA descriptor 1409 having avalue in EAID 1509 that is the same as the value EAID 2207 in the ECM.DHCTSE 627 then follows first NVSC pointer 1513 to list 1411 and looksfor a MSK NVSC 1601 that has an MSK ID field 1603 containing the samevalue as MSK ID field 2213 in the ECM. Having found such an MSK NVSC, itdetermines from no_exp_dat flag 1607 whether expiration date field 1605contains a valid time value, and if so, it compares that value with thevalue in the ECM's time stamp field 2233. If the value in time stampfield 2233 is more recent in time, DHCTSE 627 will not use MSK 1608 fromMSK NVSC 1601 to decrypt control word 2235. The secure element continuessearching for an MSK NVSC with the proper MSK ID and an unexpired MSK,and if it finds such a MSK NVSC, it uses that MSK NVSC; if it finds nosuch MSK NVSC, it does not decrypt the control word.

DHCTSE 627 similarly searches list 1411 for an entitlement bitmap NVSC1613 or an entitlement list NVSC 1623 that contains an entitlement IDthat is the same as one of the entitlement IDs 2245 in the ECM. If (1)DHCTSE 627 finds an NVSC with such an entitlement ID and (2) there is novalid expiration time in the NVSC that specifies the entitlement ID thatis earlier than time stamp 2233 in the ECM and (3) DHCTSE 627 has alsofound a valid MSK NVSC 1601 as described above, DHCTSE 627 decryptscontrol word 2235 using the MSK and decryption information 2209 in theECM. Decryption is done using the 3DES algorithm that was used toencrypt the control word. In a preferred embodiment, the control wordcontained in the ECM is a counter value as described above, and DHCTSE627 produces the control word that actually is used to decrypt theservice instance by re-encrypting the integer using the MSK and the 3DESalgorithm. That control word usable by the service decryptor is thenreturned to service decryption module 625, which uses it to decrypt theservice instance.

As is apparent from the foregoing description, when DHCTSE 627 searchesan entitlement agent's entitlement agent information 1333 for a givenentitlement for a service, it continues searching until it has eitherfound an NVSC that contains the entitlement or it has reached the end oflist 1411. What this means in logical terms is that the entitlementsthat a given entitlement agent can grant are the logical OR of theentitlements specified in entitlement agent information 1333. Forexample, if one entitlement bit map NVSC that contains the sameentitlement ID as the ECM has expired but another has not, DHCTSE 627disregards the expired NVSC, and based on the active NVSC, producescontrol word 2235.

It should further be pointed out here that time stamp 2233 in the ECMand the expiration information in the NVSCs prevent reuse of a previousmonth's MSK to decrypt an instance in the current month and also preventreuse of a previous month's entitlements in the current month toimplement the protection against replay attacks described in the Bankerand Akins patent application supra.

Where further restrictions apply to an entitlement, DHCTSE 627 searchesfor that information as well in entitlement agent information 1333. Forexample, if blackout/spotlight field 2223 of the ECM indicates that ablackout applies to the service, DHCTSE 627 uses blackout/spotlightinformation 2236 to determine whether the location specified by xcoordinate 1521 and y coordinate 1523 is within the square specified byblackout/spotlight information 2236; if so, DHCTSE 627 does not decryptcontrol word 2235. When a spotlight applies, the procedure is of coursethe opposite: DHCTSE 627 decrypts the control word only if x coordinatefield 1521 and y coordinate field 1523 specify a location within thesquare.

As previously noted, the techniques that are used to grant entitlementsaccording to geographical area may be generalized to grant entitlementsto various subsets of customers. For example, entitlements may beconceptually represented in a Venn diagram, blackout/spotlightinformation 2236 may specify an area in the Venn diagram that representsthe set of customers that are entitled to receive the service, and xcoordinate 1521 and y coordinate 1523 may specify the location of thecustomer in the Venn diagram. One use of such an arrangement would be torestrict access to an instance of a service according to a customer'sdesire that users of his or her DHCT not have access to instances withobjectionable content. In other embodiments, of course, more coordinatesor other ways of representing set membership could be used.

Event Services

When the ECM accompanies an instance of an event, interpretation of theECM takes place as described above, except that the entitlementinformation for the event is contained in an event NVSC 1701. DHCTSE 627searches the entitlement information 1333 for the entitlement agenthaving the EAID that is in the ECM for an event NVSC 1701 containing anevent descriptor 1703 with an entitlement ID 1713 that is the same asone of the entitlement IDs 2245 in the ECM. If the event is a standardpay-per-view event, DHCTSE 627 then examines the flags 1705 to determinewhether the customer has canceled the event and whether purchase of theevent has been confirmed (always the case with standard pay-per-view).The DHCTSE 627 then compares purchase time 1707 and end time 1709 withtime stamp 2233 to determine whether the time indicated by the timestamp is within the period indicated by fields 1707 and 1709. If theexamination of event NVSC 1701 indicates that the customer is entitledto the event, DHCTSE 627 decrypts control word 2235 as described above.

With IPPV or NVOD events, allow IPPV flag 2229 in the ECM must indicatethat the event is one that need not be purchased in advance. Freepreview flag 2219 may also be set to indicate that the portion of theevent instance accompanied by the ECM is part of the free preview, andcancel window flag 2231 may further be set to indicate that the eventcan still be canceled. If free preview flag 2219 is set, DHCTSE 627simply looks for a MSK NVSC 1601 in EA information 1333 that containsthe MSK specified by MSK ID 2213 in the ECM. If the DHCTSE 627 finds onethat is valid, it decrypts control word 2235.

If free preview flag 2219 is not set, DHCTSE 627 goes to the event NVSC1701 having the entitlement ID 1713 that is the same as one in ECM field2245. If flags included in flags 1705 indicate that the purchase of theevent has been confirmed and the event has not been canceled, DHCTSE 627decrypts control word 2235. If the event has not been canceled and hasnot been confirmed, but time stamp 2233 indicates a time that is withina predetermined period after purchase time 1707 indicated in eventdescriptor 1703, DHCTSE 627 also decrypts control word 2235. It is bythis means that the service instance continues to be decrypted betweenthe time the FPM is sent to the entitlement agent and the time theentitlement agent returns the acknowledge IPPV/NVOD event EMM. Thiscauses the confirmation flag to be set in flags 1705.

Cancellation of Entitlements to Events: FIGS. 17, 19, and 22

Whether a user can cancel a previously purchased entitlement to anIPPV/NVOD event that he or she has purchased preferably depends on theevent. There are three possibilities:

-   -   the entitlement can be canceled up to two minutes past purchase;    -   the event can be canceled during a period of time termed a        cancellation window, or    -   the event cannot be canceled.

Which of the three possibilities is associated with a given event isdetermined by the purchasable entitlement data 1913 in the GBAM thataccompanies the event. One flag in flags 1917 indicates whether theevent can be canceled; another indicates whether cancellation ispossible in a cancellation window. If neither flag is set, the eventcannot be canceled. When DHCTSE 627 makes an event descriptor 1703 forthe event, the values of the flags in the GBAM are used to set flags inflags 1705 which indicate whether the event may be canceled or during acancellation window only. Again, if neither flag is set, the eventcannot be canceled.

The user cancels an event by requesting cancellation via customer input628 to DHCT 333. When DHCT 333 receives the input, it provides acancellation request, including the EAID and entitlement ID for theinstance, to DHCTSE 627, which uses the EAID and the entitlement ID tolocate the event NVSC 1701 that contains event descriptor 1703 for theevent. If the flags in flags 1705 indicate that the entitlement cannotbe canceled, DHCTSE 627 indicates that fact to DHCT 333, which thenindicates that the entitlement is not cancelable to the user. If theflags indicate that the entitlement can be canceled, DHCTSE 627 simplysets the canceled flag in event descriptor 1703. If the flags indicatethat the entitlement can be canceled only during a cancellation window,and an ECM indicating the cancel window has ended has not yet beenreceived, DHCTSE 627 sets the cancel flag in event descriptor 1703;otherwise, it indicates to DHCT 333 that the entitlement cannot becanceled, and DHCT 333 so informs the user. If the event has beencanceled, DHCTSE 627 clears the acknowledged flag, which action causes anew FPM to be sent to the entitlement agent for the event. Theentitlement agent responds to the FPM by adjusting its billing asrequired by the cancellation and sending a new acknowledge EMM.

Interactive Sessions

The chief difference between broadcast services and interactive servicesis that each session of the interactive service has its own interactivesession key, which is contained in the interactive session NVSC for theinteractive session. The NVSC for the interactive session also containsthe entitlement ID for the interactive session. In an ECM thataccompanies the MPEG-2 stream for an interactive session, MSK ID field2213 is set to a value which indicates that the MPEG-2 stream is to bedecrypted using an interactive session key. When DHCTSE 627 interpretssuch an ECM, it uses entitlement ID 2245 to find the NVSC for theinteractive session and then uses the interactive session key containedin the NVSC to decrypt control word 2235.

Detailed Description of Transaction Encryption Device 603: FIGS. 24 and25

Each CAA that can authorize entitlement agents in digital broadbanddelivery system 501 and each EA that can grant entitlements in system501 has a Transaction Encryption Device or TED 603 in system 501.Preferably, each CAA or EA has its own separate TED in system 601.Alternatively, the TEDs could be combined in one device. The TED 603stores the secret keys used by the entity to which it belongs and hashardware and software to do encryption, decryption, key generation, andauthentication as required by the entity. The keys are kept secure byimplementing the TED without a user interface or user I/O devices, byimplementing it in a tamper resistant container, by connecting the TEDonly to the DNCS and using a secure link for that connection, and bykeeping the TED in a physically secure environment such as a lockedroom.

In the case of a TED 603 for a CAA, the TED 603 stores the private keyscorresponding to the three public keys representing the CAA in the DHCTs333, encrypts and provides sealed digests for of EMMs from the CAA tothe DHCTs 333, and decrypts and authenticates messages from the DHCTs333 to the CAA. In the case of a TED 603 for an EA, the EA TED does thefollowing:

-   -   (1) stores the public and private keys for the EA and the MSKs        for the EA;    -   (2) generates the EA public and private keys and the MSKs;    -   (3) encrypts and prepares sealed digests for the EMMs sent on        behalf of the EA;    -   (4) prepares the shared secret digests used to authenticate        global broadcast messages;    -   (5) provides the MSKs to SEES module 620 for use in encrypting        instances of services;    -   (6) generates interactive session keys (ISKs) for interactive        session EMMs and provides them to SEES module 620 for use in        encrypting the interactive session; and    -   (7) decrypts FPMs and other messages sent from DHCT 333 to the        entitlement agent.        TED 603 in Conditional Access System 601: FIG. 24

FIG. 24 shows the relationship between a number of TEDs 603 and the restof conditional access system 601. Portion 2401 of conditional accesssystem 601 includes a CAA TED 2427 for a CAA that authorizes entitlementagents in system 601. Portion 2401 also includes one EA TED 2425 foreach of the n+l entitlement agents which the CAA has currentlyauthorized for DHCTs 333 in digital broadband delivery system 501.Alternatively, all EA TED 2425 functions could be combined into a singleTED, which could include the CAA TED 2427 function. Each TED is kept ina physically secure area 2428 and is connected to DNCS 507 by a securehigh-speed link 2423 that connects only DNCS 507 and the TEDs 603. Inthe preferred embodiment, the secure link is a secure Ethernet link.DNCS 507 uses TED 605 to encrypt EMMs, to decrypt FPMs, to generate EApublic and private keys, to generate MSKs and ISKs, and to prepareglobal broadcast message digests. DNCS 607 has a remote procedure callinterface to the TEDs 603 for performing these operations, and,consequently, programs executing on DNCS 607 can use the facilities of aTED simply by making a procedure call.

DNCS 507 is the sole connection between a given TED 603 and the rest ofconditional access system 601. DNCS 507 is connected by a network 2415to systems belonging to the CAA and the various EAs. Each of theseentities has a database containing information relative to its function.CAA 2405 has CAA database 2403, which contains at least the CAA's threepublic keys and encrypted versions of the corresponding three privatekeys, the entitlement agent identifiers for the entitlement agents thatthe CAA authorizes, and a per-DHCT database that contains the names,types, and numbers of the NVSCs that the CAA has allocated to eachentitlement agent authorized for the DHCT.

Each EA 2409(i) has its own EA database 2407(i). EA database 2407(i)preferably contains the EAID for the EA, a list of the MSK IDs andexpiration dates for the MSKs that the EA is currently using, and adatabase of the services and/or instances that the EA is providing. Thisdatabase of services contains at least the entitlement ID for eachservice. EA database 2407(i) also includes a per-DHCT database of theentitlement IDs, entitlement expiration times, and MSK IDs for theentitlements and MSKs sent in EMMs to the DHCT. The per-DHCT databasemay also contain customer billing information such as the informationrequired to deal with the purchase information in an FPM.

Key certification authority 2413 is an entity that certifies the publickeys of DHCTs 333 to DNCS 507. In a preferred embodiment, keycertification authority 2413 is maintained by the manufacturer of DHCTs333. DHCT key database 2411 contains a database of DHCT serial numbersand their public keys. When a user of a DHCT 333 wishes to purchase aninstance of a service offered by an EA, the user sends a purchase orderto the EA with the serial number (which is also the IP address) of theDHCT 333. The EA provides the serial number to DNCS 507, which maintainsa database 2421 of DHCT public keys by serial number. If the serialnumber is not in the database, DNCS 507 sends a request for the publickey to KCA 2413. The request contains the serial number, and the keycertification authority responds to the request by sending a digitallysigned message 2412 to DNCS 507. This message contains the DHCT's publickey. DNCS 507 has the public key for the key certification authority anduses the public key and the digital signature to confirm theauthenticity of the DHCT public key in the message. If the public key isauthentic, DNCS 507 places it in public key database 2421.

DNCS 507 is further connected via another high-speed link 2417 to SEES620, which is provided with MSKs for encrypting instances of services.Additionally, DNCS 507 provides global broadcast messages (GBAMs) andEMMs for broadcast via transport link 517 to the DHCTs 333. Finally,DNCS 507 is connected via the reverse path provided by LAN interconnectdevice 617 to the DHCTs 333 and receives FPMs from the DHCTs 333. Inother embodiments, DHCT 333 may also send EMMs to DHCTs 333 by thisroute.

Data flows in portion 2401 are shown by labels on the arrows connectingthe components. Thus, an EA 2408(i) sends unencrypted contents 2410 ofEA EMMs and global broadcast messages to DNCS 507 and receivesunencrypted contents 2412 of FPMs for the EA from DNCS 507. With EA EMMsand global broadcast messages, DNCS 507 uses EA TED 2425(i) to do thenecessary encryption, digest making, and key generation and then sendsthe encrypted and authenticated EMMs and global broadcast messages, aswell as the MSKs, to SEES 620, as shown at 2426 and 2418. In the case ofEMMs, which are repeatedly sent over an extended period of time to theDHCTs, DNCS 507 stores the encrypted EMMs in EMM database 2420 andprovides them to SEES 620 from there. With FPMs, DNCS 507 uses the EATED 2425() for the EA 2409() to which the FPM is addressed to do thedecryption and authentication and sends decrypted FPM contents 2412 toEA 2409(i). DNCS 507 treats CAA EMMs the same way as EA EMMs, exceptthat the encryption and digest making is done using CAA TED 2427.

DNCS 507 also contains a database of encrypted entity information 2419,which comprises encrypted copies of the private keys and MSKs stored inthe TEDs 609 that are connected to DNCS 507. This encrypted entityinformation is used to restore a TED if a malfunction or the physicaldestruction of the TED should cause loss of the key information. Theencryption is done in the TED using a pass phrase. When the informationhas been encrypted, it is output to DNCS 507 and stored in database2419; when the TED is restored, the information is input together withthe pass phrase to the TED, which then decrypts the key information.

Detailed Implementation of TED 2425(i): FIG. 25

FIG. 25 is a detailed block diagram of a preferred embodiment of an EATED 2425(i). In the preferred embodiment, EA TED 2425(i) is implementedusing a standard computer motherboard and chassis with a standardEthernet board and additional means for accelerating RSA encryption anddecryption.

As shown in FIG. 25, the main components of TED 2425(i) are CPU 2501,memory 2505, a hardware random number generator 2537, an Ethernet board2541, and a number of RSA accelerator boards 2539(0 . . . n), allinterconnected by bus 2503. The use of more than one RSA acceleratorboard 2549 permits RSA encryption and/or decryption in parallel; inconsequence, the preferred embodiment of TED 2425(i) is capable ofencrypting a plurality of EMMs very rapidly, e.g., within a second,while also performing other operations involving encryption, digestmaking, or decryption at a similar rate.

Memory 2505 contains EA information 2507, which is the public andprivate key for the entitlement agent to which TED 2425(i) belongs, theMSKs for the EA, and code 2523, which is the code executed by CPU 2501.The parts of memory 2505 which contain code 2523 and EA information 2507are non-volatile, with the part containing code 2523 being read-only andan the part containing EA information 2507 being both readable andwritable. The code that is of interest to the present discussionincludes:

-   -   (1) MSK generating code 2525, which generates MSKs and ISKs from        random numbers provided by random number generator 2537;    -   (2) RSA key generator 2517, which generates public and private        RSA keys from random numbers;    -   (3) MD5 code 2529, which performs the MD5 one-way hash        algorithm;    -   (4) 3DES code 2531, which does 3DES encryption and decryption;    -   (5) GBAM authorization code 2533, which makes the shared-secret        digest used to authenticate global broadcast messages;    -   (6) RSA encryption/decryption code 2535, which performs RSA        encryption/decryption with the assistance of RSA hardware 2539;    -   (7) EA information encryption code 2536, which encrypts EA        information 2507 with a pass phrase for storage in DNCS 507;    -   (8) EMM code 2538, which produces encrypted and authenticated        EMMs; and    -   (9) FPM code 2540, which decrypts and checks FPMs.

EA information 2507 contains the information needed to do the encryptionand authentication of GBAMs and EMMs sent on behalf of the EArepresented by TED 2425(i). EA information 2507 also facilitates andcontains information for decryption and authenticity checking on FPMsdirected to that EA. In a preferred embodiment, EA information 2507includes at least: (1) EAID 2509, which is the EAID for EA 2409(i), EAKu 2511 and EA Kr 2513, which are the public and private keysrespectively for EA 2409(i); and (2) a MSK entry (MSKE) 2515 for eachMSK being used by EA 2409(i) in conditional access system 601 to whichTED 2425(i) belongs. Each MSKE 2515 contains MSK identifier 2517 for theMSK, the expiration time 2519, if any, for the MSK, MSK parity 2520 forthe MSK, and MSK 2521 itself.

Operations Performed by EA TED 2425(i)

When EA TED 2425(i) is initialized, it is provided with the EAID for theEA to be represented by TED 2425(i). It stores the EAID at 2509 and usesRSA key generation code 2517 and a random number from random numbergenerator 2537 to generate EA public key 2511 and EA private key 2513,which are stored in EA Information 2507. A Remote Procedure Call (RPC)permits DNCS 507 to read EA public key 2511. Other RPCs permit DNCS 507to read TED 2425(i)'s serial number, to get and set TED 2425(i)'s systemtime, and to call TED 2425(i) to determine whether it is responding. TED2425(i) responds to this call with its serial number. EA TED 2425(i)also reports a number of alarm conditions to DNCS 507. These includeencryption partial and total failure, random number generation failure,memory failure, and TED and Ethernet overload.

Continuing with the encryption and authentication of EMMs, DNCS 507 hastwo RPCs, one for EMMs generally and one for MSK EMMs. When DNCS 507 isto make a non-MSK EMM for EA 2049(i), it receives the following from EA2409(i):

-   -   (1) the serial number of the DHCT 333 which is the destination        of the EMM;    -   (2) an EAID for EA 2409(i);    -   (3) the EMM's type; and    -   (4) the information needed for an EMM of that particular type,        for example, an entitlement bit map together with the first        entitlement ID, the expiration date, and the no-expiration date        flag.

DNCS 507 uses the serial number to look up the public key for the DHCT333 in public key database 2421, uses the EAID to determine which TED2425 to use, formats the information as required for an EMM of thistype, and provides the formatted information (1123, 1125, and 1127 inFIG. 11) via the RPC to TED 2425(i) together with the DHCT's public key.EMM code 2538 then uses MD5 code 2529 to make a digest of the formattedinformation and uses RSA E/D code 2535 to encrypt the formattedinformation with the DHCT's public key and encrypt the digest withprivate key 2513 for the EA. The encrypted formatted information and theencrypted digest are provided to DNCS 507, which adds whatever else isnecessary and places the EMM in EMM database 2420.

For an MSK EMM, DNCS 507 receives the EAID, the DHCT serial number, theEMM type, the MSK parity, the MSKID, and any expiration date from EA2409(i). DNCS 507 then retrieves the DHCT serial number, formats theinformation, and makes the RPC call as just described. In this case, EMMcode 2538 looks in EA Information 2507 to find the MSK corresponding tothe MSK ID and adds the MSK to the formatted information. Then EMM code2538 uses MD5 code 2529 to make a digest of the formatted information.EMM code 2538 then uses RSA encryption/decryption code to encrypt theformatted information with the DHCT's public key and encrypt the digestwith the EA's private key and returns the EMM to DNCS 507, as describedabove.

The interface for giving a global broadcast message its authenticationinformation requires the MSKID of the MSK that is to be the sharedsecret and the contents of the global broadcast message. GBAMauthorization code 2533 in TED 2425(i) uses the MSKID to locate MSKE2525 for the MSK, combines MSK 2521 with the contents of the globalmessage (GBAM header 1807 and global broadcast data 1809 in FIG. 18),and uses MD5 code 2529 to produce the digest (GBAM MAC 1805), which itreturns to DNCS 507.

With messages sent from the DHCT 333 to the EA, such as the forwardedpurchase message, the IP packet in which the message is sent includesthe IP address of the DHCT 333 which is the source of the message, andthat in turn includes the serial number of DHCT 333. DNCS 507 uses theserial number to locate the public key for DHCT 333 in public keydatabase 2421 and provides the public key to TED 2425(i) together withencrypted envelope key 2103, CA FPM message 2105, and FPM signedauthentication 2107 from the FPM. FPM code 2540 then:

-   -   (1) uses EA public key 2511 and RSA encryption/decryption code        2535 to decrypt FPM encrypted envelope key 2103;    -   (2) uses 3DES code 2531 and the decrypted envelope key to        decrypt FPM encrypted events 2113;    -   (3) uses RSA encryption/decryption code 2535 and the public key        for DHCT 333 to decrypt FPM authentication 2107; and    -   (4) uses the decrypted encrypted events with MD5 code 2529 to        produce a new hash which it compares with the decrypted value of        FPM authentication 2107. If this comparison indicates that the        FPM is authentic, TED 2425(i) returns the decrypted events to        DNCS 507, which in turn forwards them to EA 2409(i).

The MSKs in MSK 2515 are generated by TED 2425(i). The interface for MSKgeneration simply requires the MSKID for the new MSK, the parity for thenew MSK, and any expiration time. MSK generation code 2525 receives arandom number from random number generator 2537 and uses it to generatethe new MSK. Then the MSKE 2515 for the new MSK is made and added to EAinformation 2507. If there is already an MSKE 2525 for the MSKID for thenew MSK, the new MSKE replaces the existing MSKE. TED 2425(i) alsogenerates interactive session keys for the add interactive session EMM.Key generation is as described for the MSK EMM. Once TED 2425(i) hasprovided the EMM content with the encrypted key to DNCS 507, itoverwrites the area in memory 2505 where the interactive session key wasstored.

CAA TEDs

CAA TEDs 2427 have the same hardware as EA TEDs, but in the preferredembodiment, they only encrypt the CAA EMMs used to establish anentitlement agent in a DHCT 333. EMM encryption is done exactly asdescribed for EA TEDs. The only keys required for encrypting andauthenticating CAA TEDs are the DHCT 333's public key and the CAA'sprivate key. They therefore need only store one of the threepublic-private key pairs that represent the CAA. The CAA public-privatekey pair is generated elsewhere. The private key is encrypted using apass phrase that is provided to CAA TED 2405 along with the key pair.CAA TED then decrypts the private key and stores the decrypted privatekey, but not the pass phrase, in memory 2505. The encrypted private key,but not the pass phrase, is stored in encrypted entity information 2419in DNCS 507 as well.

Authenticating Data for Applications Running on DHCT 333: FIG. 23

The foregoing has disclosed how conditional access system 601 uses theconditional access authority, the entitlement agents, DHCTSE 627, andtransaction encryption device 603 to provide security for its ownoperations and for the keys and entitlement information required todecrypt an instance of a service. Another function of conditional accesssystem 601 is that of ensuring secure data downloads for applicationsexecuting on DHCT 333. There are two paths by which data may bedownloaded: (1) in an MPEG-2 stream via the high bandwidth path runningfrom SEES 619 via transport network 517 to HFC network 521 to DHCT 333,and (2) in IP packets via the lower bandwidth path running from controlsuite 607 via LAN interconnect device 617 and QPSK modulator 621 to HFCnetwork 521 and DHCT 333.

As with the data used in conditional access system 601, there are twoaspects to the problem: security and authentication. Security may beattained by encrypting the data. In the case of data delivered by thehigh bandwidth path, encryption may be either by DES using an MSK whenthe data is intended for all DHCTs 333 having a given entitlement agentor by means of the public key for the DHCT when the data is intended fora specific DHCT 333. In the case of data delivered via the lowerbandwidth path, the data is addressed to the IP address of a specificDHCT 333 and may be encoded with the public key of the DHCT 333. In thecase of encryption with a MSK, the MSK is provided by transactionencryption device 603, and, in the case of encryption with the publickey of the DHCT 333, transaction encryption device 603 can provide thekey or do the encryption itself. DHCTSE 627 contains the keys needed todo the necessary decryption in DHCT 333.

The authenticating entities in conditional access system 601 comprisethe conditional access authority and the entitlement agents.Authentication of downloaded data is done in the same fashion as inEMMs, namely by using a one-way hash function to make a digest of thedownloaded data and then encrypting the digest with the private key ofthe authenticating entity to make a sealed digest. In the preferredembodiment, the sealed digest is made in transaction encryption device603. When the downloaded data arrives in DHCT 333, DHCTSE 627 uses thepublic key of the authenticating entity to decrypt the sealed digest andthen uses the one-way hash function to again hash the downloaded data.If the downloaded data is authentic and has not been corrupted intransit, the decrypted sealed digest and the result of hashing the datain the one-way hash function will be equal. It should be noted at thispoint that the authentication is done not by the originator of the data,but rather by a CAA or EA that is known to the digital broadbanddelivery system. Moreover, because the CAA or EA is already known toDHCT 333, downloading of authenticated data to DHCT 333 can occurwithout intervention of the user of DHCT 333.

There are many ways of relating the authentication to the data beingauthenticated. One way is to use a GBAM as described above with regardto FIG. 20. In such a case, the GBAM payload 2003 would be the digestfor the data being downloaded and entitlement agent 2005 would encryptthe digest with its private key as well as making a digest using payload2003 and a MSK. Another way is to simply send a message via the MPEG-2transport stream or using an IP packet that contained an authenticationportion as well as the data.

One kind of data that can be downloaded using the above techniques iscode to be executed by the general purpose processor in DHCT 333. Thememory used by the processor includes a portion that is flash memory.That is, the memory cannot be written to like ordinary writable memory,but can be rewritten only as a whole. Such memory is typically used tohold downloadable code. FIG. 23 shows a message containing downloadablecode. Code message 2301 has two parts: authentication part 2303 and codepart 2305. Code part 2305 contains encrypted or unencrypted code, as thesituation requires. Authentication part 2303 contains at least two itemsof information: authenticator identifier (AID) 2307 and sealed digest2309. Authenticator identifier 2307 is the CAAID or EAID for theconditional access authority or entitlement agent that is authenticatingcode 2305; sealed digest 2309 is made by hashing code 2305 in a one-wayhash function to make a digest and then encrypting the digest with theprivate key of the CAA or EA that is authenticating the code. SD 2309 isproduced in a preferred environment by a transaction encryption device605.

Code message 2301 can be sent either in a MPEG-2 transport stream or asan IP packet. Message 2301 may be broadcast to any DHCT 333 that has theauthenticating CAA or EA, or it may be sent to a specific DHCT 333. Inthat case, the packet(s) carrying code message 2301 will include anaddress for DHCT 333. In the preferred embodiment, the address is DHCT333's serial number. When code message 2301 arrives in the DHCT 333 forwhich it is intended, code executing on the processor performs theone-way hash function on code 2305 and provides the result together withAID 2307 and sealed digest 2309 to DHCTSE 627. DHCTSE 627 uses AID 2307to locate the public key for the CAA or EA and then uses the public keyto decrypt sealed digest 2309. Finally, it compares the hash value indecrypted sealed digest 2309 with that provided by the code executing onthe processor, and, if they are equal, DHCTSE 627 signals that the codehas been authenticated.

Public Key Hierarchy: FIG. 28

The various elements of the system described herein collectivelyimplement a public key hierarchy 2801 within the network. This isadvantageous because such a hierarchy can be used to establish the“trust chains” that support scaleable and spontaneous commercialinteraction between DHCTs 333 and other networks that employ publickey-based security, such as the Internet. It can also be used toestablish trust in user commercial interactions with the DBDS 501.

FIG. 28 shows the hierarchy of public key certification in the DBDS.There are two independent “trust chains” shown. On the left hand side isthe “DHCT chain,” which establishes the validity of the public keysassociated with DHCTs 333 and enables trusted use of digital signaturesmade by the DHCT 333. On the right hand side, is the “Operator chain,”which establishes the validity of public keys associated with thenetwork operators and the subtending EAs within each system and enablestrusted use of signatures of these entities.

The DHCT signature 2806 may be used as described elsewhere herein toauthenticate messages sent from the DHCT 333. However, for recipients tobe able to trust such DHCT signatures as authentic, they must know withcertainty that the public key claimed to be associated with DHCT 333 isin fact the true key which matches with the DHCT's private key. This isaccomplished by certifying the DHCT certificate 2806 with the factoryprogrammer certificate authority (FPCA) signature. The FPCA signaturecan be trusted because reference can be made to FPCA certificate 2805.The DHCT certificates 2806 and the FPCA signature as well as the FPCAcertificate 2805 are preferably made at the manufacture time of DHCT 333in a secure way. Since it may be necessary over time to issue new FPCAcertificates and use new FPCA signatures, each FPCA certificate is alsocertified with a signature of the DHCT Root which may have its owncertificate 2804. Said DHCT root certificate 2804 may either beself-signed or may be certified by another authority. DHCT rootsignature is preferably administered in a highly tamper-resistantdevice, such as one that meets the requirements of FIPS 140-1 Level 3certification.

In the operator chain, the various EA certificates 2803 are used to makesignatures in the manner described elsewhere herein. Likewise, theOperator CAA signature using the Operator CAA certificate 2802 is usedto certify each EA signature as described previously herein. Above theoperator CAA signature, two Root CAA signatures may be used to introducean operator CAA 2802 to a DHCT 333 in a secure way. In fact, preferablyat manufacture time, there are three Root CAA public keys placed intothe secure NVM of the DHCT 333. Then, authentic messages from any two ofthe Root CAAs may be used to replace the third Root CAA public key withthat of the Operator CAA whose key is certified in Operator CAAcertificates 2802. The Root CAA is preferably administered by themanufacturer in a tamper-resistant device that meets or exceeds therequirements of FIPS 140-1 Level 3 certification. It is possible,however, through an appropriate sequence of messages, to change all ofthe Root CAA public keys to be those of other CAAs that the manufacturerhas no control over. It is thus possible to remove the manufacturer fromthe signature chain. In this case, the Root CAA can be some otherorganization approved by one or more operators or it may be administeredby an operator.

As shown in FIG. 28 and described elsewhere herein, each operator mayhave a plurality of EAs. In a preferred embodiment, there is a differentEA and an associated EA certificate 2803 for every operating site of anygiven operator. This ensures that DHCTs cannot be migrated betweenoperational sites without the knowledge and participation of theoperator CAA signature 2802.

The geo-political CA certificate 2807 shown in FIG. 28, is not requiredto operate the normal conditional access and electronic activities ofthe operator. However, the operator may desire to link its signaturechain into a larger chain to be able to participate or have DHCTs 333participate in transactions involving entities outside of the operator'sDBDS. In this case, the signature chains may be readily linked to thoseof geo-political CA and its signature 2807 by having the public keys ofone or all of the DHCT root signature 2804, the Root CAA signature 2808or operator CAA signatures 2802 certified by the geo-political CAsignature. This is accomplished by having a certificate placed in adatabase for each of the public keys associated with signatures 2804,2808 and 2802. Said certificate is signed with the private key of thegeo-political CA 2807.

FIG. 29 shows an EMM generator 2901. As described elsewhere herein, itis preferred that DHCTs 333 that are operated by different operators indifferent DBDS instances are controlled by an operator CAA that isspecific to that operator and system. Since DHCTs 333 at manufacturetime are not configured to be controlled by any operator CAA, butinstead are controlled by three Root CAAs the public keys of which areplaced in the memory of the secure processor during manufacture, theymust be reconfigured for control by different operators. This must bedone securely. As described elsewhere herein, messages bearing thedigital signatures of two of the Root CAAs can be used to reconfigurethe terminal with respect to the third CAA. The EMM generator 2901 isused to produce one of the two messages needed to introduce a newOperator CAA public key in a certified way to the DHCT 333. DHCT publickey certificates 2902 are input to the EMM generator so that it may knowfor which DHCTs messages are to be made. The DHCTs that will becontrolled by a specific operator may be placed in a separate file ofthe input device or may be associated with an operator in other waysclear to those skilled in the art.

Prior to generating introductory EMMs 2903, certified public keys of thevarious operators served by the EMM Generator 2901 are loaded into thepublic key memory 2904 of the EMM Generator 2901. Thus, when EMMgenerator 2901 reads input of DHCTs needed to be introduced to OperatorA, the EMM generator uses the public key of Operator A read from memory2904 to produce EMMs containing the public key of Operator A. Likewise,prior to generating introductory EMMs 2903, the private keys of the RootCAAs must be loaded into the private key memory 2905 of the EMMgenerator 2901. Said EMMs are digitally signed by the EMM Generator 2901using the private keys of the Root CAAs contained in memory 2905. Sinceprivate signing keys are contained in memory 2905 of EMM Generator 2901,the EMM Generator 2901 must be implemented in a secure fashion thatprevents discovery of the values of the Root CAA private keys stored inmemory 2905. EMM Generator 2901 should thus be implemented in atamper-resistant device that meets the requirements of FIPS 140-1 Level3 or higher.

Since two Root CAA private keys must be used to sign separate CAAIntroductory EMMs 2903, there are preferably two EMM Generators 2901implemented, one each for each of the two Root CAA private keys. It isalso preferred that EMM generators 2901 are operated in separatephysical facilities.

The Detailed Description of a Preferred Embodiment set forth above is tobe regarded as exemplary and not restrictive, and the breadth of theinvention disclosed herein is to be determined from the claims asinterpreted with the full breadth permitted by the patent laws.

1. A method for providing an instance in a conditional access system,the method comprising the steps of: selecting a digital bit stream froma plurality of digital bit streams; encrypting the selected digital bitstream according to a first level encryption method to provide anencrypted instance; combining the encrypted instance with the pluralityof digital bit streams to provide a partially-encrypted bit stream; andtransmitting the partially-encrypted bit stream.
 2. The method of claim1, wherein each of the plurality of digital bit streams includes apacket identifier, and wherein the selecting step selects the digitalbit stream by identifying a predetermined packet identifier.
 3. Themethod of claim 2, wherein all of the selected digital bit stream isencrypted according to the first level encryption method.
 4. The methodof claim 1, wherein each of the plurality of digital bit streamsincludes a packet identifier, and wherein the selecting step selects thedigital bit stream by identifying a plurality of predetermined packetidentifiers.
 5. The method of claim 4, wherein a portion of the selecteddigital bit stream is encrypted, wherein the encrypted portion and anunencrypted portion of the selected digital stream are combined with theplurality of digital bit streams.
 6. The method of claim 5, wherein theselected digital bit stream is a program.
 7. The method of claim 5,wherein the selected digital bit stream is an elementary digital bitstream.
 8. The method of claim 1, wherein the selecting step includesselecting more than one digital bit stream from the plurality of digitalbit streams, wherein the more than one digital bit stream is identifiedby predetermined packet identifiers.
 9. The method of claim 8, whereineach of the more than one digital bit stream includes a distinct packetidentifier, wherein the selecting step selects the more than one digitalbit stream by identifying at least one of the distinct packetidentifiers.
 10. The method of claim 8, wherein a portion of each of themore than one digital bit stream is encrypted according to the firstlevel encryption method.
 11. The method of claim 8, wherein all of themore than one digital bit stream is encrypted according to the firstlevel encryption method.
 12. The method of claim 8, wherein at least oneof a portion of each of the more than one digital bit stream and all ofthe more than one digital bit stream is encrypted according to the firstlevel encryption method.
 13. A method for providing a program in aconditional access system, the method comprising the steps of: selectinga program from a transport stream; encrypting a portion of the program;combining the encrypted portion and the remaining portion of the programwith the transport stream; and transmitting the combined stream.
 14. Themethod of claim 13, wherein the program includes a plurality ofelementary bit streams, and wherein each of the plurality of elementarybit streams includes a plurality of packets, each packet having a packetheader.
 15. The method of clam 14, wherein the packet header includes apacket identifier identifying the packet as a member of a packet stream,wherein the packet identifier is indicative of at least one of a videostream, an audio stream, and a data stream.
 16. The method of claim 15,wherein the encrypted portion includes at least one of the plurality ofpackets associated with the video stream.
 17. The method of claim 16,wherein the at least one of the plurality of packets is selected by thepacket identifier indicative of the video stream.
 18. The method ofclaim 15, wherein the encrypted portion includes at least one of theplurality of packets associated with the audio stream.
 19. The method ofclaim 18, wherein the at least one of the plurality of packets isselected by the packet identifier indicative of the audio stream. 20.The method of claim 15, wherein the encrypted portion includes at leastone of the plurality of packets associated with the data stream.
 21. Themethod of claim 20, wherein the at least one of the plurality of packetsis selected by the packet identifier indicative of the data stream. 22.The method of claim 15, wherein the encrypted portion includes at leastone of the plurality of packets associated with at least one of thevideo stream, the audio stream, and the data stream.
 23. A method forproviding a program in a conditional access system, the methodcomprising the steps of: selecting a digital bit stream from a pluralityof digital bit streams; encrypting a percentage of the selected digitalbit stream; combining the encrypted percentage, the unencryptedpercentage, and the plurality of digital bit streams to provide apartially encrypted bit stream; and transmitting the partially encryptedbit stream.
 24. The method of claim 23, wherein the encrypted percentagecomprises at least one of a video packet, an audio packet, and a datapacket.
 25. The method of claim 24, wherein the at least one videopacket, audio packet, and data packet are selected by identifying apacket identifier.
 26. The method of claim 23, wherein the percentagevaries over time.
 27. The method of claim 23, wherein the percentage isselected based on content.